The higher level (based on the level, not amount of XP within a level) character gets more XP when in a group and killing mobs [and groups get more XP than solo]. It is the opposite for quests. Correct me if this is wrong, but this is what I believe to be true. So, were you're toons the same level? If they were, I never noticed this....my toons are ALWAYS out of sync due to me running five different classes (i.e., class quests...at least I assumed as much until now).

If your toons were not the same level the amount of experience given will differ (AFAIK). I usually hand in the lower level quests first (If I have multiple quests done to hand in) to get the most XP possible. Of course, I take 3 played days to level to 60 so ask some of those that only take 1 day (which I still don't know how it's done) =)