Puzzle requires mate in one

At lichess.org/training/hVDP1 I played Rxg7+ Kh8 Rxf7+, calculating Kg8 Rg7+ Kh8 Rg8#, which is forced mate in two. But that was marked as a mistake! I agree I had mate in one, but how can mate in two be bad?

The goal is to find the best moves to get to the mate, so that's why the quickest mate is rewarded.