This is a known issue. Basically, the number 64 was arbritrarily chosen as a limit to the number of records for a null search.
The reason why this limitation is imposed is that when there are a large number of users in the directory, null searches can cause CPU spikes on the web server performing the search, which in most cases is a CallManager server. This of course can produce undesirable results.
The eventual goal is to make this hard limit a configurable number, however that capability does not yet exist.
If someone wants to browse a user directory without limiting the search then as an alternative, they can go to http://callmanager/ccmuser/directory.asp where callmanager is the CallManager server name.