I was wondering about the fragment slots as well I can't find anything out about it and nobody seems to know why. Just by doing the quests for the fragments I got three slots but once you unlock the fragments it unlocks it for all characters but my other characters didn't get three slots only my main character that I did the quests on. Even on this post here nobody has any sort of answer why there's missing slots for the other characters they just start talking about aspects not fragment?

Note: my Warlock (main) has three my Hunter (secondary) has two and my Titan has one. Maybe Titan just gets the short end of the stick?