You can earn the Custodian, Reviewer, and Steward badges for seven different review types. If you earn one such badge, then it does not appear on the unearned badges page, even if you don't have that badge for all review types.
The tag badges can be earned for any tag, but if you earn one, it remains on the unearned badges page. This seems inconsistent. Is this because the system sees a bronze homework badge to be different from a bronze calculus badge? Shouldn't a tag badge appear on the unearned badges page only if you are yet to earn one?