Disfonctionning of the sort order on last comment timestamp for a "tracker-like" view

I'm migrating my site from 5.x to 6.x. After having converted my view with the packaged tool and manually finished the migration path, I've got a strange SQL query (see the ORDER BY string below) concerning the sort.
I've compared the settings of my view with those of the default tracker view, but I haven't found differences for the settings of the sort criteria.
Any idea ?

Thanks for your great work !
Best regards.

The View code is the following :

>
$view = new view;
$view->name = 'contributions_recentes';
$view->description = 'Montrer les nouvelles contributions sur les blogs et les forums.';
$view->tag = '';
$view->view_php = '';
$view->base_table = 'node';
$view->is_cacheable = FALSE;
$view->api_version = 2;
$view->disabled = FALSE; /* Edit this to true to make a default view disabled initially */
$handler = $view->new_display('default', 'Defaults', 'default');
$handler->override_option('fields', array(
  'type' => array(
    'id' => 'type',
    'table' => 'node',
    'field' => 'type',
    'label' => 'Type',
  ),
  'title' => array(
    'id' => 'title',
    'table' => 'node',
    'field' => 'title',
    'label' => 'Titre',
  ),
  'name' => array(
    'label' => 'Name',
    'alter' => array(
      'alter_text' => 0,
      'text' => '',
      'make_link' => 0,
      'path' => '',
      'alt' => '',
      'prefix' => '',
      'suffix' => '',
      'help' => '',
      'trim' => 0,
      'max_length' => '',
      'word_boundary' => 1,
      'ellipsis' => 1,
      'html' => 0,
    ),
    'link_to_user' => 1,
    'exclude' => 0,
    'id' => 'name',
    'table' => 'users',
    'field' => 'name',
    'relationship' => 'none',
  ),
  'comment_count' => array(
    'label' => 'Réponses',
    'set_precision' => FALSE,
    'precision' => 0,
    'decimal' => '.',
    'separator' => '',
    'prefix' => '',
    'suffix' => '',
    'exclude' => 0,
    'id' => 'comment_count',
    'table' => 'node_comment_statistics',
    'field' => 'comment_count',
    'relationship' => 'none',
  ),
  'last_comment_timestamp' => array(
    'label' => 'Dernier commentaire',
    'date_format' => 'small',
    'custom_date_format' => '',
    'exclude' => 0,
    'id' => 'last_comment_timestamp',
    'table' => 'node_comment_statistics',
    'field' => 'last_comment_timestamp',
    'relationship' => 'none',
  ),
  'timestamp' => array(
    'label' => '',
    'alter' => array(
      'alter_text' => 0,
      'text' => '',
      'make_link' => 0,
      'path' => '',
      'alt' => '',
      'prefix' => '',
      'suffix' => '',
      'help' => '',
      'trim' => 0,
      'max_length' => '',
      'word_boundary' => 1,
      'ellipsis' => 1,
      'html' => 0,
    ),
    'link_to_node' => 0,
    'comments' => 1,
    'exclude' => 0,
    'id' => 'timestamp',
    'table' => 'history_user',
    'field' => 'timestamp',
    'relationship' => 'none',
  ),
  'new_comments' => array(
    'label' => '',
    'alter' => array(
      'alter_text' => 0,
      'text' => '',
      'make_link' => 0,
      'path' => '',
      'alt' => '',
      'prefix' => '',
      'suffix' => '',
      'help' => '',
      'trim' => 0,
      'max_length' => '',
      'word_boundary' => 1,
      'ellipsis' => 1,
      'html' => 0,
    ),
    'set_precision' => FALSE,
    'precision' => 0,
    'decimal' => '.',
    'separator' => '',
    'prefix' => '',
    'suffix' => '',
    'link_to_comment' => 1,
    'no_empty' => 1,
    'exclude' => 0,
    'id' => 'new_comments',
    'table' => 'node',
    'field' => 'new_comments',
    'relationship' => 'none',
  ),
));
$handler->override_option('sorts', array(
  'last_comment_timestamp' => array(
    'order' => 'DESC',
    'granularity' => 'second',
    'id' => 'last_comment_timestamp',
    'table' => 'node_comment_statistics',
    'field' => 'last_comment_timestamp',
    'relationship' => 'none',
  ),
));
$handler->override_option('filters', array(
  'status' => array(
    'operator' => '=',
    'value' => '1',
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'status',
    'table' => 'node',
    'field' => 'status',
    'relationship' => 'none',
  ),
  'type' => array(
    'operator' => 'in',
    'value' => array(
      'blog' => 'blog',
      'forum' => 'forum',
      'article' => 'article',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'type',
    'table' => 'node',
    'field' => 'type',
    'relationship' => 'none',
  ),
));
$handler->override_option('access', array(
  'type' => 'role',
  'role' => array(
    '2' => '2',
  ),
  'perm' => '',
));
$handler->override_option('title', 'Les contributions récentes');
$handler->override_option('header_format', '4');
$handler->override_option('footer_format', '4');
$handler->override_option('empty_format', '4');
$handler->override_option('use_pager', '1');
$handler->override_option('style_plugin', 'table');
$handler->override_option('style_options', array(
  'columns' => array(),
  'default' => 'last_comment_timestamp',
  'info' => array(
    'last_comment_timestamp' => array(
      'sortable' => TRUE,
    ),
  ),
  'override' => FALSE,
  'order' => 'asc',
));
$handler = $view->new_display('page', 'Page', 'page_1');
$handler->override_option('path', 'view/contributions_recentes');
$handler->override_option('menu', array(
  'type' => 'normal',
  'title' => 'Contributions récentes',
  'description' => '',
  'weight' => 0,
  'name' => 'navigation',
));
$handler->override_option('tab_options', array(
  'type' => 'none',
  'title' => '',
  'description' => '',
  'weight' => '0',
));

The SQL query is the following :

SELECT node.nid AS nid, node.type AS node_type, node.title AS node_title, users.name AS users_name, users.uid AS users_uid, node_comment_statistics.comment_count AS node_comment_statistics_comment_count, node_comment_statistics.last_comment_timestamp AS node_comment_statistics_last_comment_timestamp, history_user.timestamp AS history_user_timestamp, history_user.nid AS history_user_nid, node.created AS node_created, node.changed AS node_changed FROM node node INNER JOIN users users ON node.uid = users.uid LEFT JOIN node_comment_statistics node_comment_statistics ON node.nid = node_comment_statistics.nid LEFT JOIN history history_user ON node.nid = history_user.nid AND history_user.uid = ***CURRENT_USER*** WHERE (node.status <> 0) AND (node.type in ('blog', 'forum', 'article')) ORDER BY node_comment_statistics_last_comment_timestamp ASC, node_comment_statistics_last_comment_timestamp DESC