It is fairly common for people to have different prescription in each eyes. Just like,some people have one foot slghtly larger than the other. One eye slightly bigger than the other. Make sure to wear glasses which are prescribed especially for your eyes. Particularly when wearing contact lenses, don't wrongly wear your left contact lens on your right eye, or wear your right contact lens on your left eye, or will damage your vision.
Just like the both hands, one is exercised more than the other, so it is normal that one is stronger than the other. It may because the brain find more difficult to make the both eyes focus at the same time. So the brain control the dominant eye to focus more often and then formed the different prescription in the two eyes.
I think it is caused by the eye dominance.You know one of our eyes tend to be dominant,and for most people it is the right eye.Our brain usually receive signal from the right eyes.It is the same as our left and right hand.Usually the right hand is used frequently.