When you look in the mirror you see your face flipped left to right. What you don't see is your skin turned inside-out. That would be horrific! Now if we take the gorgeous Suzanne (Monkey) and tab into Edit Mode, and then enable the Normal direction indicators:
we see all faces and vertices are facing outward:
Now, if I chop off half of the mesh, set the Pivot Center to 3D Cursor:
and select all A. Then duplicate shift+D and mirror ctrl+M along the X axis, the faces and vertices on the mirrored half are facing inward:
This could be a programing error, or a bug, or maybe theres a very good reason for it. It can be quite irritating to have to keep flipping the Normals:
I know that the Mirror Modifier doesn't have this problem, but what about ctrl+M, can anyone please enlighten me on this?