That is how experience works for opponents that you use the Charm spell on, but your creature and summons give you full experience. I had my (mage) creature summoned while I was approaching the main wyvern nest on Sentinel Island, and had the experience message pop up a couple times without even seeing the wyvern (another couple times I caught part of the blood cloud from when they were killed).
Charmed opponents give the same proportion of experience as the damage that you (or your creature / summons) did to that opponent compared to damage done by the rest of the opponents while it was charmed. Charm is best used on opponents summoned by enemy summoners, since they do not give experience anyway.
