The count says one protected square, so the zugzwang works and your king really gets in. But from here it is still not a win. The verdict on a6 depends on which side your king attacks from. Come down the far side, as in this position, and Black holds. Only if your king could march up the rook's file, to a4 and a5, would the same pawn lose. With the pawn back on a7, White always wins.