Must be a bug then, because it seems mad to get penalised for being lower level, surely the opposite should apply. It's all T3 scenario's too, so like say i have 75k heals, 10k damage, 1 DB,etc. Another guy, high 20's, has say 40k heals, 8k damage, no DB's. His xp is double.
You will probably find though that the XP he gets, while double yours, has less impact as he is much higher level. In percentage of level terms, you probably get more.