Funny you should say that about the numbers being the same unchartered at the support group I go to there is a fairly even split girls to boys and I have often wondered about it.
I think you are right about the girls being described as shy, I also think a big part is that girls often arent physically aggressive at school, when frustrated or anxious they seem to be more likely to shut down than lash out.
Consequently they are not causing anybody any trouble and therefore go unnoticed. Dd3 used to work extremely hard at being invisible at her old school but vented everything from the day on me within seconds of coming out of the classroom at 3.30