That's because a mate in two is quite often an easy combination in the head's calculations. You see the opposing king in danger...and you calculate out the moves.
Whereas sudden mate in one situations are often missed because we do not expect them or look for them very proactively.
Remember that Kramnik (or which GM was it) missed a mate in one?
This is how often I miss checkmate in 1 vs. checkmate in 2 in my rapid games. Could anyone explain why this might be happening or how to fix it?