I think it is because boys tend to be more laid-back and don’t like to show their working. In recent years, with the introduction of the grades 1 – 9 to replace the letter grades, exams have become more rigorous to combat grade inflation.
More than one in four exam entries by girls aged 16 received top grades of A or 7 and above in this summer’s exams in England, Wales and Northern Ireland, an improvement of half a percentage point to 25.3% compared with last year, while just 18.6% of entries by boys in Year 11 achieved the same grades.
Guardian Education 22nd August 2019