I have seen pitchers develop new pitches over the winter that change their overall performance. I have seen pitchers that had a great fastball and rise in summer ball, then try to add a screw, cruve and drop that messed up the other pitches. High School coaches, from what I've seen, out a high premium on hitting spots. Sometimes that causes a pitcher to 'guide' the ball instead of a regular windup and release. That an affect both speed and accuracy. That's my two cents worth.