Bogus "form_build_id_/nodetype/" variable created on type settings page

I have an hook_form_alter() function:

function announcement_form_alter(&$form, $form_state, $form_id) {

// echo "announcement_form_alter(&$form, $form_state, $form_id) called <br>\n";

  if ($form_id == 'node_type_form' &&   // for node_type_form
     // only for announcement
     $form['#node_type']->type == 'announcement' && 
      isset($form['identity']['type'])) { //  may not be needed

// dprint_r($form);

    $form['announcement_block_max_list_count'] = array(
      '#type' => 'textfield',
      '#title'  => t('Maximum number of block announcements'),
      '#default_value' =>
  variable_get('announcement_block_max_list_count_announcement', 3),
      '#description'   => t('Maximum number of items in block'),
      '#required'      => FALSE,
      '#weight'        => 0
    );

    $form['announcement_display_classification'] = array(
      '#type' => 'checkbox',
      '#title'         => t('Display announcement classification'),
      '#default_value' =>
variable_get('announcement_display_classification_announcement', 1),
      '#description'   =>
        t('Insert the classification in the announcement modules'),
      '#required'      => FALSE,
      '#weight'        => 0
    );
  }
}

Variables announcement_block_max_list_count_announcement and
announcement_display_classification_announcement are now correctly stored and retrieved.

However, one strange variable form_build_id_announcement
is created in table variable:

select * from variable where name like '%announcement%';

| announcement_display_classification_announcement | i:1;
| comment_form_location_announcement | s:1:"0";
| announcement_block_max_list_count_announcement | s:2:"30";
| comment_subject_field_announcement | s:1:"1";
| comment_preview_announcement | s:1:"1";
| comment_controls_announcement | s:1:"3";
| comment_anonymous_announcement | i:0;
| comment_default_per_page_announcement | s:2:"50";
| comment_default_order_announcement | s:1:"1";
| comment_default_mode_announcement | s:1:"4";

| form_build_id_announcement | s:37:"form-d5383d0c1653760c25e871b5c4feed6e";

| comment_announcement | s:1:"2";
| upload_announcement | s:1:"1";
| nodeapi_example_announcement | s:1:"0";
| node_options_announcement

Toshi