There are a number of subtleties to this demonstration. For example, the image of the candle is doubled due to the fact that the glass window pane has two surfaces. The distance between the images is twice the thickness of the glass divided by the index of refraction of the glass. However, by far the most important point is that the image is not on the surface of the mirror, as many people think. I emphasize this point by distinguishing between a reflection and an image.
Reflection is something that happens at the surface of the mirror.
An image of a real object in a plane mirror is in that Alice Through The Looking Glass space behind the mirror. There are several other important points to make as well. But the ones I treat as most important can be summarized as follows. The image is behind the mirror, not on its surface. And, of course, other points can be made depending upon the audience.