It's written down there ==> "Changing Tiers"
Your average is from a certain no. of the best results in your current tier, 9 hole results being doubled.
So, if you have imaginary scores of 70 (or 35), 72 (or 36), 74, 76, 78, 80 in the mix of six scores counting, the resulting average is 75. Playing a 77 now will replace the 80, thus dropping the average by 3 strokes difference / 6 rounds = 0.5, i.e. to 76.5.
Thus, a score above the average can push it down. Your 39 (think absolute scores!) is 78, it will have replaced a 79 or 80, good for you. Second good news is that your average will only drop from now on, only scores of 78 (for now) and better counting in.