this is not really a math question...
Experience is not a pie that you just cut up and adds to 100%. Experience is not just action, it's knowledge as well.
You and me are walking along, and we walk into an alleyway and some drug dealer pulls out a switchblade and jumps us. If you end up being the guy that wrenches it from his wrist, knees him in the crotch, and elbow him to the head... while I've done nothing... you've gained the following 'experience'
1) Be wary of people in Alleys
2) People can carry weapons and hurt you
3) Close Combat (disarming)
4) Hitting people in the head knocks em out...
Now, if I was there watching you do that, I've learned the traits 1, 2, and 4. I did not learn #3 really, but I have some idea.
My point is that there's some 'easy knowledge' that's shared and duplicates itself to fit outside the pie. If 50 people were with us and 1 person did that, there would be 49 people who've realized 1, 2, and 4, and you realized all 4 of them. There wouldn't be 50 people who gained 2% of the experience that was gained at that fight (i.e. alleys can be bad...)
That's why the xp in groups SHOULD naturally add up to more then 100% of the monsters experience value. You can't really do a perfect 6 way split unless you believe that action is the sole determiner of experience, and then you have no choice but to alllocate experience INDIVIDUALLY even in teams (i.e. like Dungeon Siege does)