A lot depends on how you play each toon.
If you are VIP, each toon gets Bonus XP each day. This is used on quests as well as on mob kills. So you could play for less time each day on some toons than others. For example, if you play one toon only while there is bonus XP available, then they will level faster as far as XP is concerned and need less quests to complete a level than a toon that you continue to play after the bonus XP is used up.
And then there is crafting XP. You earn xp while gathering mats and doing any crafting function. If the champ does not gather and/or craft and the others do, then the others will collect more XP while doing the quests. They will thus need to do less quests to complete the level.
And if one toon kills more mobs than the other toons, then they will collect more XP while doing the same quests.
And if the champ solos while the other toons group, it will take him longer to level up. Being in a group earns you bonuses for mob kills so a solo player might get 100 xp for the exact same mob as a group player gets 150 xp.
So as you can see, there are so many ways to level and it sounds like you are just playing them differently.
[CENTER][B][I][COLOR=yellow]LOTRO proves that Middle Earth is Hobbit Forming![/COLOR][/I][/B][/CENTER]