[SirAlvarex]

The official explanation states that the percentage of XP is split by the total amount of XP you've gained to level.

I'll take a crack at showing some mathematical examples


So if a level 8 druid has 8,000 XP, and a level 10 SK has 14,000 XP, on a kill of a mob worth 100XP it is dividied up like:

8000+14000 = 22000 total XP in group
Druid is 0.363636363636364 of the share
SK is 0.636363636363636 of the share

If 100 XP is rewarded, add the 2% bonus for being in a group.

102XP total:
102 * .363 = 37.026
102 * .636 = 64.872

So now given two characters of the same level, with a normalized leveling curve of 1,000xp per level:

level 10 druid 10,000xp
level 10 sk 14,000xp

24000 total group XP
Druid is 0.416666666666667 of the share
SK is 0.583333333333333 of the share

Assuming 100xp mob normalized to 102xp for the group:
Druid: 42.432
SK: 59.466

Druid's next level is at 11,000, or 24 kills
SK's next level is at 15,400, or 24 kills

They level at the same rate

Now, lets put that druid halfway through his level.

Druid 10,500
SK 14,000
24,500 total group XP
Druid is 0.428571428571429 share
SK is 0.571428571428571 share

102XP mob:
Druid: 43.656
SK: 58.242
Druid needs 11.45 kills to get 500xp to level
SK needs 24.13 kills to get 1400xp to level.

See how the curve isn't exactly a straight line? By the Druid being halfway through the level at the moment they started grouping together, he started getting "his bigger share" much earlier in the level. That means that he is further ahead of his curve and will level slightly faster than his partner.

If both partners start at the exact same point in the level as eachother (to the infinitismal digit), they will level at the exact same rate.