Wrong array key name being used in validate helper

Taken from IRC:
Hello need some help with an issue I am looking at. I am trying to figure out how install.php works. and here I see something I do not understand. Drupal 7-dev HEAD.
In install.php line 311 you will find $form['advanced_options']['db_prefix'] = array(
now if I am right the variable name will be db_prefix. while in line 348 it is being validated: if (!empty($database['prefix'])how can form variable names change ? Is this something I do not know? I am not sure.

Now Heine and Berdir also think this is a bug.
Here are the full codes as were referred above:
insall.php lines 309 - 317:

    // Table prefix
    $db_prefix = ($profile == 'default') ? 'drupal_' : $profile . '_';
    $form['advanced_options']['db_prefix'] = array(
      '#type' => 'textfield',
      '#title' => st('Table prefix'),
      '#default_value' => '',
      '#size' => 45,
      '#description' => st('If more than one application will be sharing this database, enter a table prefix such as %prefix for your @drupal site here.', array('@drupal' => drupal_install_profile_name(), '%prefix' => $db_prefix)),
    );

install.php, lines 348 - 350 Iinside validate helper):

  if (!empty($database['prefix']) && is_string($database['prefix']) && !preg_match('/^[A-Za-z0-9_.]+$/', $database['dprefix'])) {
    form_set_error('db_prefix', st('The database table prefix you have entered, %db_prefix, is invalid. The table prefix can only contain alphanumeric characters, periods, or underscores.', array('%db_prefix' => $db_prefix)), 'error');
  }

I will submit a patch if you think this is actually a bug. I am still too novice to tell that :P