I mean I could write why but then I wouldn't finish so-
There can really only be one drop per area per method of play.
But you could totally count a fire clockwork tunnels as different from an ice one, or a T3 one as different from a T2 one, or even box drops as different from enemy drops.
The gist of this is that when multiple areas drop the same thing, that unless they are all exactly even in payout, one comes out as dominant, and players just go for that one area to farm at.
This makes the player hunt down specific levels and areas to farm at