The mirror thing ...
sort of makes sense (at least to a warped mind like mine).
Think about projecting a image onto a sheet, like shadow puppets, but from both sides of the sheet. Then viewing the sheet from the other side, the projection is reversed. The screen and the normal camera video capture sensor is on the one side of the sheet and the front facing camera is on the other side.
However, it does seem that HTC could have taken care of this in the software.