Magic mirrors can never be trusted.
It is true that they will answer any question with a true image.
But the user is responsible for asking the right question, and for interpreting the image.
The mirror can lie most completely while still strictly showing nothing but the truth. The wrong picture of the true thing can lead to the most unfortunate conclusions.
The old witch was getting sloppy in her old age, tending to forget things she had set out to do. She asked her mirror to remind her, and it usually did. But it faithfully reminded her of exactly what she asked, not inferring anything about the requests.
As a result, she was prepared for a different gentleman than the one that knocked on the door.
He wasn’t sure what to expect, but what answered the door was not it.