Anxiety has a really disproportionate impact on math skills (well, on their demonstration, in performance metrics) relative to any other academic skills
Secondly girls are more prone to math anxiety, and thirdly, the impact of anxiety on them is apparently greater, too, for reasons that aren't clear.
My own DD is a great example of this (albeit at a post-secondary level)-- she can (and does) know calculus material well enough that she's a desirable study buddy even for those students who are seeing the material for a second time, and frankly runs circles around them in terms of her conceptual understanding, but they handily outscore her on assessments, and particularly on exams.
It's a maddening problem. We have figured that we have to shave about 30% off the top of her logical expected performance, even under pretty good conditions. Really not kidding.
Knowing your stuff isn't the same thing as showing that you do, which is a separate skill, and one that is incredibly sensitive to anxiety, particularly in girls and particularly in mathematics.
I can look up some references for this one, but they're pretty easy to find on G-scholar.
