Well, you can't replace tub on TT01/02 without custom diff cases, as they're integrated into chassis. I think that's point, when TT02 becomes something different.
But you're right, Tamiya has sometimes weird logic in their chassis names. In case of TA05 they went by TA05 bulkheads = TA05 chassis (with exception of MS). On the other hand, TB03,04,05 and EVO6 share same bulkheads and difference between 03 and 04 is similar to TA05 and ver.2...