That's true. Even if the average man is taller than the average woman, there's still some women who are taller than some men. How many, depend on how large the difference in average is, and what the standard deviance is.
Thus, biology could only exaplain a 90/10 split if either the difference in average skill was very large, or the standard-deviance was very small. It seems exceedingly unlikely that either is the case for general math-aptitude or related technical skills.