Before having played XX rounds your average is a real average. XX is a number of ranked rounds which depends on your tier (it's 400 for legend and 50 for tour master for example). So before having played XX rounds, the average is just the sum of all your scores divided by the number of rounds (scores from 9-holes rounds are converted into 18-holes scores by multiplying them by 2).
Once you have played XX rounds or more, the average is computed by taking into account only your best ever XX rounds.
Let's take an example with XX = 4. You have played 4 rounds (two 9-holes and two 18-holes) with the following scores:
32, 68, 34, 72.
First you convert them into 18-holes scores:
64, 68, 68, 72.
Your average is then (64+68+68+72)/4 = 68. Now you have reached this critical XX number. You play 1 more round and score 70. This score is above your current average but once you have played XX rounds or more, you have to take only into account your best ever XX rounds. In this example, this means your best ever 4 rounds. So the score of 70 will replace the score of 72. Your average is then
(64+68+68+70)/4 = 67.5
So a WGT average can decrease even if one is shooting above his average.
However, once you have played at least XX rounds, your average can no longer decrease increase* since you take into account only your XX best ever scores.
Regards
*Thanks Jim I am a bit dyslexic sometime. When I passed my license driver examination, they asked me to cover my left eye for a test and I covered my right eye :) .