It is a bug, but the whole xp system needs to be tweaked. Right now, as far as I can tell...
1)Mission completion does not award any XP.
2)Killing a mob gives the same xp regardless of level. Kill a level 1 mob at level 1...get 7 xp. Kill a level 1 mob at level 35, get 7xp.
3) All mobs seem to grant xp based solely on their level, and not on the rank (Like you mentioned).
4) Kill a mob on a team, split xp. Level 1 mob killed on a team of 2, grants 4 xp(I am assuming it is 3.5 rounded up). There should be some sort of grouping bonus to reward team play. Otherwise, a team of two killing things twice as fast, accumulates the same xp in the same amount of time as someone doing it solo.
