I believe there was a bug once that when you opened a certain chest or package it would change everytime you'd re-open it. So that leads me to believe the loot is generated when a creature dies. Rather then when it spawns. But that's just a guess.
Back in TBC, loot from raid bosses were generated when the raid ID was created. There was an exploit to determine whether certain items were on a boss before killing any of them. If not, people would just disband and reform raid to create a new ID. Blizzard fixed the exploit, but kept the loot table being determined at mob spawn. This system was still in effect into WotLK.
Unless they changed it, loot is generated when the mobs spawn.
Last edited by Aquamonkey; 2013-06-14 at 08:00 PM.
Yes it's generated when spawned. One of the ex-gm AMA's on reddit had that question asked and answered during it. He also said gm's sometimes will float invisibly over raid teams going for world first kills and watch them as they plant their face into the brick wall of progression. Gotta love it lol.