Hopefully someone else might give a more precise technical explanation but its basically the way the mirror is figured. If it was 'correct' the light would focus in a sharp point. Undershoot the mark and its undercorrected, overshoot the mark and its overcorrected.
The startest link below shows how to check for it.
http://www.astunit.com/tutorials/startest.htm
Be nice to have a perfectly 'correct' mirror