You are actually doing a lot of things here including a shorter backswing length. When you grip down on the club the club and stand closer to the ball, therefore effectively shortening your arms, the radius of your arc is smaller and therefore a shorter backswing length. Tempo could be the same but by standing closer to the ball you probably are not making much of a turn or at least a very narrow one. Not that what you are doing is uncommon but I only use it for shots very close to the green.