The nicotine ratings you're referencing aren't empirically measured like is sometimes down on dip labels. It's just based on whatever the site owner or user-generated info people are going off of.
I think
-nicotine effect on various people is quite variable
-some descriptions are wildly wrong, just based on tradition or reputation
One thing I've noticed is that blends with very strong flavor tend to be rated artificially high on the nicotine scale. I like high-nicotine tobacco: some of the heavier blends I've smoked are the G&H twists, G&H Dark Flake, 1792 Flake, etc. Most people talk about the nicotine level on these, and they are right.
Yet a blend like Dunhill Nightcap, I've smoked quite a few times, and have never felt any nicotine at all. Yet when you read reviews, you'll see people going on about how strong it is nicotine wise. (For the record, I actually felt the tiniest bit of nic buz puffing away at PS Luxury Bullseye Flake on an empty stomach. But not with Nightcap!)
Basically, I think things with strong flavor (most often a lot of latakia) are rated higher than they deserve, just because the full flavor leads one to believe it is a "strong" tobacco. This is why the black twists by the Gawith companies are so often said to be stronger than the brown twists, when the opposite is actually true. The more the tobacco is stoved, the more nicotine is removed from the leaf! Yet people often remark that the black twists are stronger than the brown twist, when it is in fact the exact same twist just stoved longer.
I also agree with deathmetal--flakes are quite strong, probably partly because of the density. You have to think about how much tobacco you are smoking through. I think this is part of the reason Five Brothers is strong. True, it's nothing like flake, quite the opposite, but it burns so darn quickly that you're going through a lot of tobacco really fast.