this is one of those bugs that blizz is supposed to fix "some time soon", and who know how long that will be. but according to a blues post (can't find the actual post right now) if you have a character in your group that is 5 or more levels above (not sure about under) the group "should" get no xp bonus. but as of right now this has not been fixed so you can have an 80 in a group with 4 level 10's and run them through DM and they will get 3x exp bonus. this is supposedly going to be fixed but it is on there "soon" list so it could be fixed tomorrow or some time next year or even later...

Hope this helps some. ^_^

~YYF