Well, I figure it was a combination of the following:
1. Sunako doesn't look into a real mirror very often, so she doesn't know what she looks like, roughly.
2. She was a very good actress who managed to imitate Sunako's poses so completely that Sunako really thought it was her reflection.
3. Frosted glass (yes, really) to hide other discrepancies, and a great deal of suspension of belief.