I think the logic is that if the ball is going into your top right hand corner and you're diving to your right, your left hand is logically higher than your right so why try to get it with your right hand which is lower. It seems perfectly logical to me - just that we've been seeing the opposite method used for so long that we thing it's normal. In ten years time people will look back at old videos and wonder what the goalkeepers were fucking about at.