'higher' than 20/20 simply means that your personal best corrected visual acuity is better than the ability to resolve 1 arcminute at 20 feet, or that you see at 20 feet what a 'normal' person sees at 20 feet. It is not unusual for many patients to be refracted to 20/16 and sometimes better (so the patient can see at 20 feet what a 'normal' person must move to 16 feet to see clearly. Due to individual physiological differences , not everyone can be refracted to 20/20. The doctor simply gave you the prescription you require, and you are lucky enough to see 'better' than 20/20. There are no side effects to speak of in your case, other than you have very good vision.
AA