trying to figure out rarity of snes games

on digital press R ratings are confusing. I know DP has been studying this a long time. Its not related to money or even complete games its supposed to be just related to cart availability vs scarcity.

Yet some games like "magic boy" are rated a "4" on DP and in my collectors guide even rated highly and yet i buy it on ebay sealed for $8.

Other games like chrono trigger are an R2, again saying that carts are fairly available, not sealed games.

Last nite I picked up a Shien's Revenge on ebay factory sealed for $80. its an R7 which is the highest R rating I have in the whole collection. Why isn't it more money? The collectors guide says its rare too.

Is it that some games are so popular, chrono, harvest whatever, that they are grabbed up immediately by collectors driving up the price but some others, altho rare, are not sought after much?

Would appreciate any help here