Tatical puzzle error

http://en.lichess.org/training/13492

I feel as if this tatical puzzle has an error. It bases the move off of the fact that in the game this was taken from, black blunders after whites move. You are looking for the best move and I would say Qc6 is the best move for its attacking to hung pawns, but the computer says this is wrong because after white plays the "correct" move the opponent blunders. This puzzle is misleading. the game it is taken from is a horrible game. both opponents played terrible.

black does not blunder :
after Qb7, white threatens mate in g7, black needs to defend.

Frapator is right. White's move is a double attack on the rook on a8 on the g7 square. The only way of stopping the mate is by moving the rook on f7. But the f8 rook is overwhelmed (it defends the a8 rook but it has to move to defend against the mate threat). The puzzle is correct.