In the old days it was believed to be that the leader of the party raid influences the loot tables to such a point that you could get different loot.

Like say, one guild in Molten Core in the old days always had like 2/3/4 loot items that literally never ever dropped if there was always the same raid leader. Ofcourse it's not the only factor so it was otherwise mostly random. So many guilds tried to enter with different raid leaders from time to time, and oddly, many guilds reported the drops dropping that hadn't dropped in the 3/4 months before.

Odd!

Anyhow, this is all speculation. As far as blue has told us, loot is determined upon entering an instance based on the raidID amongst other things. Other then that we know nothing really. ;p