It's because Lesnar is improving at a scary pace. The guy has been thrown to the wolves and has done really well.
Now, do I think he'd beat Mir if they were to rematch? Yes, because Mir isn't that good of a fighter, and I don't think Brock would make the same mistake he did last time.
He won't, however, beat Nogueira. Nog's still a great fighter and will tap Brock when they face each other. Nogueira has more ways to win, and will be there till the very end.