Why the point which I didn't focus on is sharper than the one I focused on?

I have Nikon D7000, when I focus on an object (using AF-S, Single point AF, Matrix Metering, 35mm),
I noticed after I review the picture that the point which is not in focus is sharper than the point (eye) that I focus intentionally.
I took a picture to a girl and I position focus point on her right eye but the result is the left eye is sharper! (I didn't use the point in the center of view finder)

I think you've answered your own question with your last comment (I didn't use the point in the center of the view finder).

Always use the single point dead center. Put that directly on the area to be focused, the eye in your example.

Always shoot several shots of your subject. Lenses are not perfect and don't always get it quite right.
I find that when doing portrait work, especially of children, that as many shots as the child will allow gives me a much better over all result. If the child will put up with 50 that's what I'll do… Perhaps even 100.

Point is a single shot is never a good idea because if it doesn't work correctly you've no alternative and you're stuck with that shot. If you'll shoot many you'll usually find at least one that is just what you want.

When focus is critical, do it manually. Make sure the diopter on your camera's viewfinder is calibrated properly to your sight.