Creature Handler Archive

I am 1-2-2-3 in ch and wanted to know why I just get 640 xp per rancor with a Cowardly Gurrek. I bring out my mount, a lesser plains bol and he gets 1060 xp per rancor. Why is this? Should I use a different creature to attack or should I be getting xp somewhere else? What do you CH's suggest. Thanks in advance.