As a master your average is computed from your best 40 ranked rounds. If you have played more than 40 ranked rounds, your score will either remain static or go down, if you have played less than 40 RR's your score will fluctuate until you have reached that number.
If you make a score that is lower than one of the 40 that goes to make up your average, that old score will be replaced by the new lower score, and your average will go down.
If you make a score that is equal to or higher than one of the 40 scores that go to make up your average, then your average will remain the same.