What is potentially even worse is not knowing which ones have been registered for the achievement system. Sometimes you simply forgot to register one, other times it could be bugged and doesn't count when you summon it.
If/when they fix this panel, if they are feeling ambitious, they should also make it possible to search for a specific name, and update the Achievement UI to show all possible pets so you know what you are missing.
The easiest solution (temporarily, since i would like sorting options or some kind of sorting very much too), would be to just check for the char variables that define availability of said pets/abilities. But if you see how they coded the way achievements are registered, it seems even with the npc helpers they can not check certain variables. For example common sense would seem to check, if a char has a certain quest status or certain pets available to summon (should be some kind of array anyway), do a loop with a check if it is registered already and done. Quest chains might be more complicated, but all info is already stored in the quest log variables. If i got a quest listed as completed and one objective was to kill Boss XY, then the achievement system should automatically "know" that i killed that boss...
I assume they probably wrote all/most of their achievement update scripts/code by hand, meaning they had to specifically account for each item that would update an achievement. Doing it this way would be a tremendous effort to get all the related info and possible cases from the game.
It seems they just didn't have the time/man-power to write scripts/code to cover all those possible cases of proof.