Difference between $form_id and $form['#id']?

Question

What's the difference between $form_id and $form['#id'] in hook_form_alter() ? Why they have not the same values?

Answer 1

Next should make sense:

<form action="somepath" id="search-theme-form"> <!-- id = $form['#id'] -->
  <input type="hidden" name="form_id" id="edit-search-theme-form" value="search_theme_form"/> <!-- value = $form_id -->
</form>

Terms:

#form['#id'] = store html form attribute id:

Used to populate form elements' id property. In rare cases, you can set this value yourself on a form element, to override the default setting.

$form_id - point to Drupal form function name, in html it's form hidden element:

The unique string identifying the desired form. If a function with that name exists, it is called to build the form array.