A legend's average is counted from the best 500 ranked scores in his tier (excluding Par3 + Par5 rounds). You have a total of 991 rounds in all tiers - perhaps there aren't 500 Legend rounds to count? This would explain that your average wobbles up and down still.
A TM has 50 best rounds to count - much quicker saturation. Certain scores (above 66?) won't move his average at all if saturated.