Because the system is fucked, simple as that.
What he said. One T4 keep we took with a guild group, the guy that was AFK the entire keep raid came #1 on the contribution.
I think it has something to do with being in a group so the system gives you all the same amount (or around the same amount) of points for participating, atleast I think it's because of that as it only happend to me while being grouped and walking to a keep while my group was already fighting quite a while there.