Another thought is that, at 6 and 7, there is a real difference in the way in which Maths and English are taught.
There is an immediacy to Maths, e.g. 'Put up your hand if you know what 2+2 equals.' Ability can be demonstrated in the classroom in front of everyone. Writing simple equations '2+2=4' is straightforward and children ca mark each other's work. The children will know who is good at Maths and will hear teachers say that those children are clever.
Learning to read, however, doesn't happen demonstrably in front of everyone, so no one knows who is best at it. Girls are usually have neater handwriting as their fine motor skills develop quicker, but we don't say they're clever for that. We say they have neat, beautiful handwriting, which conforms to the stereotype of girl's appearance being the important thing. It's much later that the content of a child's writing is judged rather than the letter formation, etc.
I distinctly remember the 'clever' kids in my year 3 class in primary school were 3 boys who were good at maths. It was only when I got to year 6 that I found out I was clever too, and I was best at English.