Their corrected vision improves of course, that's the whole point. The surgery improves or eliminates the refractive error (caused by the shape of the cornea). But this process has to cut or shave the cornea (internally or externally depending on LASIK vs. PRK). The cutting and subsequent healing can leave a little scare tissue...which interferes with light passive. It's almost impossible to do the surgery without at least a little degradation, although you may not notice it. Other folks lose one or even two lines on the eye chart. I know one airline guy who was so badly damaged that he lost his medical and became a career sim instructor. they said he needed corneal transplants but the medical plan wouldn't pay.

Imagine a camera telephoto lens...if the image is blurry you can adjust the refraction by twisting the knob. But if the lens is scratched up with sand, no amount of refractive adjustment is going to give you a perfect image.