Hi Steve,
Welcome to the forum (fancy meeting you here!)
I'm sure others will jump in but for the most part TDS is a bogus measurement. The discrepancies you saw were probably testing errors at the pool store or dealer who did the test. If you drain and refill they can sell you more chems faster! Since your spa should be drained and refilled about every 3 months (depending on bather load) it is NOT an important measurement at all!