Preprocess functions not being registered

This may be a documentation issue, but I cannot for the life of me figure out why neither THEME_preprocess_search_block_form or ENGINE_preprocess_search_block_form is not being registered. template_preprocess_search_block_form from search.module is working, but the other functions one would think might be operative here aren't.

To duplicate, add the following to your template.php under garland with the search block enabled:

function garland_preprocess_search_block_form(&$variables) {
  print "Hello world, from the search block";
}

function garland_preprocess_page(&$variables) {
  print "Hello world, from the page";
}

Clear cache or otherwise rebuild the registry. One will fire, the other won't. Curious...