The mirror returns a present image to a mind full of previous versions.

You look at a current face with a mind holding every previous one. That is why mirrors can be jarring in a way photographs are not, and why the gap seems to open suddenly rather than gradually. The image updates continuously and your internal picture does not.

When the reflection surprises you, it may help to know the surprise is about the lag rather than about the face. Looking at photographs from several ages at once tends to smooth it out. You are seeing a sequence, and the mirror only ever shows the last frame.