Probably a misunderstanding of the ENTER calculation 10% rule. ENTER is calculated from adding the study scores of your three best subjects and English (mainstream, ESL, language or lit) together, and then adding 10% of your next best two. If you still have 6 subjects after dropping one, then there's really no penalty - and more likely a gain from the increased time you have actually. Obviously no penalty too if you're just swapping subjects. However, if you now only have 5 left, it's hard to say whether you'll lose ENTER or not since you've now got more time for your primary four for which you reap 100% of the study score points, but in essence you've lost the potential 10% of the study score for this 6th subject if you don't take into account changes in your other subjects.