Just 18 per cent of media studies students in Wales got an A-grade in last week's results, compared to 44 per cent of maths students.
But that doesn't mean maths is easier - only that more able students took the subject in the first place, according to the WJEC, the Welsh exams board.
Its head of research, Raymond Tongue, has been looking at A-level candidates' attainment at GCSE, based on average points scores.
The result is a series of graphs for each A-level subject, showing the "typical" percentage of entrants at each of 10 ability ranges for GCSE.
For example, usually more than half of 18-year-olds taking maths A-level are ranked in the top two ability groups - meaning they already have a clutch of A*A-grade GCSEs.
However, in media studies, under 10 per cent of entrants are classed among the highest attainers at GCSE - which helps explain the lower number of A-grade passes. Subjects similar to maths, with many very able entrants, include modern languages and sciences. PE, like media studies, has fewer entrants of high ability. Art and drama, however, seem to attract students with a wide range of prior attainment at GCSE.
Mr Tongue said knowing such patterns for each subject would help to ensure A-level standards are maintained. For example, more media studies students achieving A-grades with no increase in entries from high attainers at GCSE would suggest the exam was getting easier.
The graphs also help keep WJEC subject officers aware of changes in their entry cohort.
"For example, if it suddenly became unpopular to do maths and all the more able pupils went and did media studies, then the exam results would change but it would be the cohort, not the exam," said Mr Tongue.
But he said the statistics give a broad picture only, and that individuals can buck the trends.
"Just because you have an all-round ability doesn't mean to say you will do well in a particular subject," he said.
"It's possible for people in the bottom categories for GCSEs to do well in A-level art."
Caroline Morgan, assistant senior manager and head of maths at St David's Catholic sixth-form college in Cardiff, believes the WJEC's new modular maths syllabus will result in more A-grade A-levels.
This year's students were the first to complete the full two-year programme, and more than half of the college's 67 maths entrants gained an A-grade last week. Miss Morgan believes the new syllabus has made the subject much more accessible to students by breaking up the pure maths elements.
"When they had a lot of pure maths, the girls especially used to get a bit muddled up and panic," she said. "Now it's spread out, so students can be examined in the bit they have just been taught and not have to do lots of pure maths all together. It builds up their confidence."