Not an optical engineer myself, but I would say it's because when the scope is not in it's optical center, the erector tube is at an angle. When this is true, and you hold the scope up to the mirror, you are looking through the scope at a slight angle at the mirror. Thus you see the actual reticle and the reflection of the reticle separately. When the erector tube is adjusted to be straight (scope is optically centered), you are looking the the scope at the mirror straight on, and thus you don't see the reticle's reflection as it is directly behind the reticle.
