How many you are under or over par IS NOT YOUR SCORE. Under or over par is a measure of your score relative to par.
2 under at St. Andrews is a score of 70, since par on St Andrews is 72.
70 is more than 67.67
So your scoring average goes up if your scoring average is not yet saturated at your tier.
At the Master tier it takes 40 ranked round scores to saturate your scoring average. Once saturated, your scoring average will no longer go up. It will stay the same if you post a score higher than your average and it your average will do down with score equal to or lower than your average.
When you advance to the Tour Master tier your scoring average will reset. Advancement to the TM tier requires a scoring average of 63.00 or lower. If your first round as a TM is 35 for 9 holes or is a 70 for 18 holes your TM scoring average will go up to 70.