rotated square do not mach the grid diagonals..how this is possible ??

 Hello Everyone !

I came across very strange issue.. (or maybe there is something i have done wrong ?..)

I have set up the guide lines 5mm on 5mm, then i have drawn square 100mm on 100mm , i duplicate it, align its center with the first one, then rotate it minus 45 degress.. then that happened.. side does not mach the diagonals of a grid.. ?! i dont understand what is wrong ?? It works on paper how can it be that it does not work here ?? Please help !! Thank YOU