Every new tier you attain, you're avg resets. I know for a fact without checking your score history, that you hit a 34 as your first game as Tour Master, that would be why you're average is 68, if on your next game you hit a 30, it'll drop to 64, more ranked games you play, the less the movement. Law of averages and all that.
Immunity for Tour Master is 50 ranked games as previously stated, this is the cut-off for Legend, from there it'll only ever go down, and as WGT works, only your 50 best ranked rounds will account for your average until you hit legend where I wouldn't have a clue.
I.e. 49 rounds of 30 = 60 average, with one round of 120 = average of 61.2, 60.99 being the movement to legend, however if on your 51st round you hit a 100, you'll have an average of 60.8 instead of 61.9 (the 120 and 100) the 120 is nulled and 100 replaces it.