Different XP gain? [Possible Bug]

Creatures seem to have different XP gains - even when they are the same level.

There doesn’t seem to be any reason for the disparity.
Fallen Carnage is a +10 Gene Strength (received second most XP)
Raven Acolyte is +3 Gene Strength (received the most XP)

Also in many cases this changes from fight to fight. The XP gained seems very arbitary.