Uneven xp gain?

I’ve been wondering why my creatures seem to get different amounts of xp. Even if a party is all the same race, and all started leveling at the same time, they seem to wind up leveling unevenly even if i periodicaly catch up the ones that are lagging behind.

There doesn’t seem to be any rhyme or reason to which ones lag and which ones are ahead. Its defferent every time i even them out.