Thanks for taking interest. So the disclaimer here would be that these are relative to themselves and it's entirely subjective and personal. I rank NES difficulty compared to other NES experiences, etc.
As for the math, it's not the best system. I'm no mathlete, but the rankings do end up sorting in a way that I find reasonable to my experience. My rankings are on a 1-10 scale. For example, I gave Double Dragon 3 a 10 because it was a very tough and truthfully a mostly awful experience (but YAY I finally beat it!). GameFaq users think it's a 4.17 (their scale goes to only 5). It's just an average of the two ((10+4.17)/2). This also means that the theoretical highest score a game can get on my scale is 7.5 which isn't ideal for round number OCD but I've grown used to it. I can probably normalize it so that 7.5 = 10 and maybe I will. There's probably a hundred better ways to do it but that's why I brought it up - to see if others do this as well and to inquire how.
I beat Dark Link using the exploit... but not knowingly. It was what I did when I got there - it wasn't until later that I learned I took the coward's way out. Oh well. The other 99% of the game was still pretty tough for me. Conversely, Contra is 17th on my list. I used to think it was hard, but a little practice went a long way and now it's no issue. It's all going to bank on personal experience and when I played it (even 5 years ago versus now- I'm probably better now).
I can't stress enough that the list is personal so I wouldn't try to convince anyone that LOZ2 will be harder than Batman - but it certainly was in my experience. The purpose of this from my perspective is more or less a self-maintained achievement list.