Think about what TA measures. Basically, it is the titratable H+ ions. And what has more of those, the H+ associated with casein (and to a smaller part other proteins), or whey and all its small amounts of salts, proteins, etc? Casein has a ton more. So, when you titrate, the whey will be rather dramatically lower. You can sometimes go .2 at gel cutting, and .12 in whey. Doesn't mean acid development has stopped or changed. You're just measuring different things. It's also why generally pH is easier, though the meters can be a PITA.