static or drupal_static in drupal_html_css() ?

Here we see:

function drupal_html_class($class) {
  // The output of this function will never change, so this uses a normal
  // static instead of drupal_static().
  static $classes = array();
//...
}

so can some one make the comment clear?
why say output of this function will never change, does it mean that
other function that use drupal_static have changing output?

can some one give us an example to explain it.
Thank you in advance.