When using propagate masks for CGeometry_Zdepth with reflection, the returned distance is the distance from the reflecting surface (i.e. the mirror) to the reflected surface (whatever is reflected).
This however isn't a useful value and unlikely the value people using this feature are interested in.
What you really want to return is distance from camera to reflecting surface plus distance from reflecting surface to reflected surface, since that's the value you need if you for example want to correctly render depth-of-field effects in reflections (in the same way as you want distance from camera to refractive surface plus distance from refractive surface to refracted surface in order to properly render depth-of-field for objects behind windows)