It's because the adequate and tall Salty doesn't give pitchers a low enough target- that's my theory. Mauer is a very big catcher, and early in his career my recollection is that some pitchers expressed concern about pitching to such a big catcher. Some felt that umps couldn't get a good look at the low pitch. Mauer, when healthy, compensates for his size by getting very low. IMO, Salty is giving too high a target and/or he is not giving the ump a good look at the low pitch.