In World of Warcraft why am I constantly falling behind my teammates in exp?

I play World of Warcraft with my two friends. I am a priest and my friends play a Warrior and a Shaman. We always do the same quests together with the exception of the class specific or profession quests. Taking that into account, I always seem to end up slightly behind them in experience. It has happened at least 4 times w/o any obvious reason. Are healers rewarded less experience or something?