I don't think lefties' pitches move any more than righties. Remember when you were a little leaguer, the pitches didn't seem like they were rainbows, they seemed to be coming straight in. The parents in the stands knew they were rainbows but we didn't. The mind takes the pitch it is used to seeing and processes it as pretty close to a straight line. We all get used to seeing right handers and having the ball tail in on us a bit. Our minds process that as being straight just as we process the fastball which actually drops a bit as being level. When we see a leftie's pitch tailing away from us our mind processes that as being twice as much movement as it really is because we're used to the ball tailing in on us. Same thing happens with a LH hitter except he sees that lefty pitch as tailing in on him big time.