“girls are less likely to take [physics] unless they're really good”
Almost certainly something like this. Can’t remember where I read it recently, will have to check my Goodreads ... but girls tend to have more choice of A levels because they have better, broader GCSEs. The research in question had looked at why students with good GCSE maths did, or did not, go on to do it at A level. There were NO boys in the population studied who had a top maths GCSE result who had not gone on to do it at A level. There were plenty of girls - but they also had good results in other subjects, and/or preferred other subjects. So you would expect with maths, and I would expect the same to be true of physics, that girls taking it would generally be VERY good and/or really passionate about the subject.