Haha. Yeah, any self-proclaimed "rarity guide" is a load of horseshit, plain and simple.
I mean, you have to think-- who came up with these numbers? Was it just some random guy that did a search on eBay one day for all the TurboGrafx games and gave a rating of "common" to any game he saw more than 10 copies for sale of, and a rating of "rarity 5" (or whatever) to games that happened to only have 2 copies up for sale at that moment? Or did they send someone over to Hudson Japan to review the production run records for each and every game and devise their list based on that? Are these numbers based on any kind of actual information? Highly unlikely.
If you have questions about a particular game, just ask around these parts. Most of us know what's actually rare and what isn't.
New Adventure Island, for example, isn't any rarer than Air Zonk. Which is to say-- not rare.