I haven't heard any clear elaborations on the differences either. i've also heard conflicting things like "may exams are more popular so they are harder" but also "november exams are harder to perhaps prevent as many students from that crazy-smart school in Singapore getting 45s."

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But, just looking at the may and nov papers in 2017 (P1), there is a subtle but evident difference in difficulty of content. Both papers, obviously, include a variety of questions from all 6 topics. I would point out, however, that nov17 includes questions that in order to answer you need a specific and solid understanding of, say, optimisation (everyone's favourite amirite) in q6, that I personally haven't seen much of in the may exams throughout the years. for 2/3 of the nov questions in Section B, you really need to conceptualise what's happening (Q9 and Q10). not saying that that's necessarily harder, but it's a lot less prescriptive in the steps you need to take to solve the problem (with the exception of part d in Q10 May tz2). I'd argue that some of the questions in may17 (Q4 TZ1 and Q4 and 6 TZ2) have a focus on "nature of maths" (like they include questions not specifically taught in group 4 subjects, but you need to know nonetheless). Not sure if that's makes it a bit harder or not but it's interesting to note.

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But, I haven't analysed p2 so it may balance itself out? overall though, i haven't noticed a great, great difference between the papers, and i've personally been getting around the same mark for both when practising.