Barring any explanation that depends upon a phenomenon outside of your own mind, the remaining explanation of the epiphenomenon is that it is a mild hallucination.
(We might be tempted to call this a "mirror hallucination", however that term is already used to refer to a hallucination where we see ourselves somewhere other than in a mirror, so is probably best avoided).
Dr. Oliver Sacks argues in his recent book, Hallucinations, that hallucinations are more common than often supposed, because they are under-reported due to stigma or fear of being considered as having a serious mental condition. This under-reporting in turn adds to the misconception that hallucinations only happen with serious mental conditions (or heavy intoxication), and so on, in a cycle that maintains this under-reporting. According to Sacks, having mild hallucinations need not be particularly troublesome.
However, it remains that hallucinations can indeed be caused by psychological or neurological complaints, so if the experience has only begun recently, or if it increases, it could well be worth seeking medical advice.