I was on RarityGuide.com, under the Dreamcast section. Some of the 2001 NTSC-U titles I understand their 'Rarity'. It was because of lower and later release counts. But how are things like King of Fighters '99 Dream Match more rare than other Launch, or early months, games?

Many of the Launch Titles seem to indicate a rather high rarity compared to something as simple as Coaster Works or 102 Dalmatians.

Is it just scarcity, or does the amount of successful transactions factor in?

Basically, what is the basis of this determination?

I looked over the FAQ and forums, but they are a mess over there.