Appreciate your answers , thanks .
I realize I made a mistake thinking that this was bug.
Seamed like a paradox to me , having a plane that is out of focus (blurred) that reflects things focused.
It is a very unusual and interesting phenomenon and lots of people find it confusing as far as I could see on photography forums.
Well , maybe this post can help someone as confused as I was (before your answers) :)
Thanks for opening my eyes and happy rendering :P