Having 20/20 vision does not necessarily mean you have perfect vision.
If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. 20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet.
