Q:Why do some subjects get sh*t scaling?
A:With some subjects, more people get high marks, so a mark of say 80 in that subject could mean the person is in the bottom half of the that group. While in some other subjects where the marking is tough, 80 could well mean the person is in the top 10% of the group. And this is one of the major reasons why one 80 mark is better than another 80.
UAI is competitive, to get a good UAI, You need to choose those subjects where you are better than others at. Getting 80s or even 90s means nothing when others also get the same mark.