Uninstall may delete other module variables and don't clear variables cache

If there is a module named colorbox_extra you will delete the other modules variables. Additional to this only variable delete will also remove the variable from the variable cache.

/**
 * Implements hook_uninstall().
 */
function colorbox_uninstall() {
  db_query("DELETE FROM {variable} WHERE name LIKE 'colorbox_%'");
}

Please uninstall variables with variable_del() only.