It's possible to have Scarlet's Skull in the same room as Red, the NPC who is searching for her. Under most circumstances, Scarlet will be the 80th Skull to collect, but the game's developers took some careful measures to try and prevent her Skull from being the one in the same room as him. However, if you collect the other Skulls in the Tower of Fate, such that Scarlet's Skull is the 80th or 81st, the remaining Skull in the same room as Red is forced to be Scarlet's Skull.