Hello hythe. This is because your stroke average (kind of equivalent to your handicap) resets to only take into account what you've done in your current tier (i.e. Master).
You've played one round at MAster (par 72), so your average score at Master is 72. If you play a second round and score 70, you'll be averaged to 71, and so on. How well you score now at Master, consistently, will determine how quickly you tier up to the next.
Hope this makes sense.
See you on the green stuff.
Rob