The supplementary exams are in general a tad tougher than normal exams everywhere.
The reasons are

1. You are getting a second chance , so they want only really deserving candidate to go through.
2. Since supplementary exams are mostly in isolation , i.e mean you don’t have tension for other subjects’ exam. So to be fair to students who studied for all the subjects at once, this is set to be tougher so that to score similar marks ,guy attempting supplementary have to work harder.