I believe that when the whole athena boosting came about they shortly there after changed the xp that you would get.

(Prot pally runs through instance and grabs mobs but doesnt attack or do dmg while toon being boosted waits at the front, then once it gets back to the front mage enters instance, pally lays concencrate and then leaves group, and then leaves instance mage then aoe's the mobs down. With that the toon being boosted got full xp from the kills"

WIth that I am pretty sure that I read a few blue posts about them nerfing this, so I am sure that its affecting said xp your getting if they did.