As a legend your score is an average of your best 500 ranked rounds. If you have not yet played 500 ranked rounds your score will go up or down as you complete each ranked round. Once you reach the magic 500 your score will either go down or remain stationary dependant on your score for each round.
If you score better than one of those 500 rounds, your average will go down, sometimes by an amount so small it will not show on your score, for example .002, but it does count so do not despair. If you score worse than all of the 500 best rounds your average will not change, but at least it won't go up.