Server side on a first come first serve method.

Unless someone else can prove it wrong, the moment someone "rolls" they generate the number which means the next 3-7 people who roll after that, cannot generate the same number (which is why we never have duplicate rolls).

That, or the moment an item drops, the game calculates a hidden number for us that we only see when we roll, but it was already previously chosen for us.

A good example would be when I hit "need" on an item in maps and got a 99, but they were still discussing if we should split the take (first day Elpis maps) and the others were like "ok, ok, roll!" but I already knew all their rolls were pointless, because I hit the 99 the moment it popped up.