Currently the idea is to have the first playthrough of each level on the early levels to have the same loot per level just to allow players to build up supplies easily. If you replay the levels then the loot will be randomized.
Then on the later levels all loot will always be randomized even on the first playthrough, so if you don't bring enough equipment into a large level for example, then you may not find enough equipment to get through the optional doors.