The distribution of marks in English (as it is the only compulsory subject so it has just about every student in the state) is used as the common scale which forms the baseline for scaling.
Let's suppose the statewide average raw mark (not the realigned HSC mark you see on your certificate) is 50% for English.
Of that entire English cohort, there would be some that also did say Chemistry. Let's suppose the average raw mark of those Chemistry students in English was 60%, which is higher than the statewide average of 50%. This suggests that there are stronger students doing Chemistry on average. Therefore, the marks in the Chemistry subject would be 'scaled up' so to speak to reflect this 'skewed' distribution.
However, if the average raw mark of these Chemistry students in English was say 40% instead then this suggests that there are weaker students doing Chemistry on average. Therefore, the marks would be 'scaled down' to reflect it skewing the other way.
This is a simplified version of how scaling works. Hopefully it makes sense and you can see why the difficulty of a course does not directly impact how it is scaled.