My guess is it's all about trained muscles that make eyes do microscopic movements.

Eyes can see only things that move, and to see a static picture there are special muscles that make retina move all the time the eyes are open (that's why blinking or closing eyes for short time gives them rest and helps to relax).

People that have these muscles more developed and move faster are able to notice and comprehend more visual information in shorter times.

Playing games and being all the time on the watch out for every pixel on the screen makes these muscles to work out and become stronger and faster.

Yep, it's just a workout.