It's natural. If you pull 3 groups, chances are the next morning one will be up to 20, the other will only be one or two levels up, and the third will be lucky to gain a single level.
It all has something to do with the amount of damage that the mobs do to you as well as the order in which they do it.
Best thing you can do is wake up in the middle of the night to kill a creature in the group that's gaining the most levels. That'll drop it's damage enough for another group to start gaining the kill credit and leveling up. If that's not feasible, no biggie, just extends your DL time by a month or two=p
If one group is doing more dmg than another group, it has a better chance to get the last hit. The group that gets the last hit gets the exp from the kill. Once a group starts lvling, it will do even more dmg and get the last hit even more, effectively robbing the other group from being able to get exp. One group levels while the other doesn't. To avoid this, groups must be the same size with relatively similar group composition. 3 ranger char will completely out-dmg 3 necro charr for example :-)