Focus points

Hi all! I just got a thought here. The black dots in the view finder of my 550D indicates the placement of the focus points. However the dots does are not the focus actual points. When I take a picture of a ruler or something similar at an angle, with quite large aperture it seems like the actual focus point is just above the black dot. I use center point only. It could be front or back focus, but I have focused on corners of doors, tables etc., and it seems that the focus point is a tad above the black dot.

Is this just a user error, or have anyone else experienced something like this?