The bug

1.21.2's new rarity system appears to have a classification oversight with respect to mob heads. Of all the head types dropped by mobs, wither skeleton skulls are the easiest to obtain, due to being an uncommon drop from player kills, while all others require a charged creeper blast. Despite this, wither skeleton skulls are considered rarer than the rest.

Expected results

Other heads (zombie, creeper, piglin) would be considered Rare at the very least, since they're harder to obtain than wither skulls.

Actual results

They are considered more common.