Your average is a function of your total ranked rounds at a given tier, Legend in your case. As the number of ranked rounds rises, the effect of a single round on your average diminishes. A high score won't increase your average much at all and likewise neither will a low score lower your average very much.
At a certain point, your average will "saturate" and scores above your average won't affect it, and scores below it will lower it gradually, usually in increments of .01.