There have been discussions on the rarity value for some items and whether they need to be adjusted.  It might not be apparent, but the rarity value is adjusted for some items over time.  If you look at the following page, this is indicated by the red and green colors under the rarity column:

Rulebooks Index

It's important to remember that just because something is rarer than something else, doesn't mean that it's more desirable and therefore more expensive.