In the API reference (http://api.drupal.org/), I was looking for a taxonomy function and so typed 'taxonomy_' into the AJAX field for Drupal 4.7.
I knew there was a function called 'taxonomy_select_nodes' which wasn't in the resulting list. However, when I continued to type (i.e. 'taxonomy_s'), the taxonomy_select_nodes showed up where it didn't before.
The 'taxonomy_save_vocabulary' function also showed up in the new list when it didn't appear in the first try.
So, basically, the AJAX doesn't seem to be returning the full list of available functions as you type in the request.