As far as I can see the proportion of entries getting A* in those subjects in 2019, which I assume was a fairly normal year, were as follows:
Maths: 16.8%
English Lit: 9.1%
Physics: 9.0%
Sociology: 4.9%
Psychology: 4.4%*
*
The average for all subjects was 7.9%
Why that is is hard to say. Is maths A level easier than psychology? Are the people taking maths A level generally brighter than the ones taking psychology? Do they work harder? Is the quality of teaching better? Are those taking maths A level at an advantage as they have studied the subject for 14 years rather than (usually) only 2? A combination of factors? *
*
The top grades should be equally difficult to get in any subject. But obviously people have individual strengths and weaknesses which mean that they might be capable of an A* in some subjects but not others!
Interestingly, in some subjects over half the entries were awarded at least an A. Most are minority languages, and presumably the vast majority of those who entered would have been native speakers. The subjects with the highest proportion of top grades were:
Russian: 87.9%
Italian: 73%
Greek (ancient): 79.2%
Polish: 59.4%
Latin: 65.4%
Further maths: 54.1%
Across all subjects: 25.8% of entries were awarded at least an A. Those with the fewest top grades include:*
*
Statistics: 9%
Film Studies: 10.7%
English Language: 11.6%
General Studies: 12.2%
English Language & Literature: 12.8%
Business Studies: 14.8%
DT: 16.4%
PE: 16.6%
Law: 16.7%
Psychology: 17.0%
Drama: 18.4%
Sociology: 18.5%
NB: for English Literature, 25.3% got at least an A, so a big difference between that and the two other English A levels.
For maths it was 41.4% so a big difference between that and statistics.
Whatever happens this year I imagine it will go back to normal next year.