Unfortunately no-one, not even Bucks CC Admissions, really know the boy girl split until it turns up - hence the problem with boys' Admissions in South Bucks this year.
However, for reference, this was the split of all those who took the Bucks 11+ over the last few years. I am useless at posting in "Code" to make a table, but this is Girls, followed by Boys:
And no, the figure of 3717 boys in 2007 & 2009 is not a typo - it was exactly the same number! The figures are very consistent, as are the figures for the Bucks resident cohort for boys and girls, and so are the numbers of boys vs. girls qualifying. Last year was a very average year for girls in every way. I suspect that your theory may be correct - that for some reason there are more boys than girls in the south of the County at present, for no good reason - something in the water 10 years ago, maybe?
However we won't know for sure until March 1st unfortunately.
To answer jellybaby's question, there were 5432 children in Year 6 at Bucks state schools, a very similar number to last year. Undoubtedly the private school numbers will be fairly constant too, although there may be more prep school children who are taking the 11+ because of the recession.
Nif is correct - standardisation is done on age alone - gender standardisation disappeared some years ago.