hook_schema_alter is called on own module install as well

Following a simple example taken from Pro Drupal Development Second Edition:

<?php
/**
* Implementation of hook_install().
*/
function markednode_install() {
  $field = array(
    'type' => 'int',
    'unsigned' => TRUE,
    'not null' => TRUE,
    'default' => 0,
    'initial' => 0, // Sets initial value for preexisting nodes.
    'description' => t('Whether the node has been marked by the
      markednode module.'),
  );
  // Create a regular index called 'marked' on the field named 'marked'.
  $keys['indexes'] = array(
    'marked' => array('marked')
  );
  $ret = array(); // Results of the SQL calls will be stored here.
  db_add_field($ret, 'node', 'marked', $field, $keys);
}

/**
* Implementation of hook_schema_alter(). We alter $schema by reference.
*
* @param $schema
* The system-wide schema collected by drupal_get_schema().
*/
function markednode_schema_alter(&$schema) {
  // Add field to existing schema.
  $schema['node']['fields']['marked'] = array(
    'type' => 'int',
    'unsigned' => TRUE,
    'not null' => TRUE,
    'default' => 0,
    'description' => t('Whether the node has been marked by the
      markednode module.'),
  );
}

?>

The former is accomplished with hook_install, which install first the extended table.

The latter with hook_schema_alter, which cast an exception since the field already exists.

I would appreciate to know if I am doing something wrong, but I'm not sure about this drupal inconsistency, on an install, a module should invoke all the _schema_alter hooks.. EXCEPTING own hook.

Does it makes sense ?