The stats are used very misleadingly. Some boys underachieve, some don't. Some girls achieve highly, some don't. Generally, girls only significantly outperform boys in literacy, and always have. The 'gaps' in achievement in maths and science are barely enough to be considered noteworthy.
This has been so since records began. There are quotes from the 1800s stating that boys were just deeper thinkers or were less show-offy than girls, which, reading between the lines meant that the girls were apparently performing better. Learning styles have varied wildly across that time, and yet it remains fairly static.
There are much, much bigger gaps when results are considered in terms of race or socioeconomic background and yet it is gender which we pay attention to. There are also bugger variations within gender groups than between them.
Also, access has ha a lot to do with it. For example, for years Physics was considered a male subject and very few females took it. However, those females that did achieved highly. However, it is only now that there are lots of girls taking the subject that people have started bleating about boys underachievement in that area.
What exactly are 'male' learning styles? There is no evidence to suggest that boys perform better in certain types of assessments or with certain approaches to teaching. And what if using these so-called boy-friendly approaches disadvantages the girls? Is that acceptable? Similarly, why do we not take up those things which have shown to improve girls learning? For example, studies have shown that where single-sex classes were introduced into schools to try and address boys underachievement, it had no impact on the boys achievement, but the girls actually improved, yet none of these schools kept these approaches up.
I believe, like several others have said, a lot of it comes down to expectation. A girl takes charge and tries to organise everyone else, she is bossy and this is discouraged. A boy does it and he is a natural leader and encouraged. Girls are definitely encouraged to do activities in their early years which set them in better stead for schooling (craft, reading, beads etc) than boys, who are expected to be running about and being boisterous.
The shools where the gap is the smallest are the ones who treat achievement as an individual issue, not working on gender stereotypes and trying to enable every child to achieve their full potential, regardless of their gender.