I have a long reply I would love to make, but I'll keep it as short as I can.
From what I've read, girls and boys at the same diagnostic level of autism (designated HFA/Asperger's for discussion's sake), present symptoms in ways unique to the inherent differences in many males vs females. So where boys may show delayed speech, girls aren't. Boys may have a restricted interest to the point of standing out (the example I read is that of a 9 volt battery), where a girl's restricted interest may be fairy tales and princes, which is perceived as normal and not as focused.
This isn't a broad application, just one to explain why girls seem to fall through the proverbial cracks.
Personally, I think it is because too many supposedly credible therapists, teachers, and psychologists still ascribe to the belief that Asperger's is a male only disorder, and assume that an 'odd' female child is just 'odd', or has other issues. In other words, a case of someone working outside of their scope of practice and/or education, which I abhor.