The other night there was a server reboot, I was killing mobs in Red Ridge - and I got a specific green item. After the reboot - didn't get that green.

Think about it in terms of server maint. If you generate the loot on mob spawn, you have to keep track of what the loot is for the life of the mob. This means a new table in the database. If you generate the loot on when the player loots the mob ... you don't need to track anything. Both results, over time, would show the same level of randomness - but generating drops on loot is more efficient.

This would explain why the loots are different.

Also, we had a reset once when my raid was in MC. We got a different drop. Messed up our DKP system at the time too, because the raid tracker f'd up and charged both the old and new recipient.