For the Moonies with Sunnie names:

Could it be they were orphaned in the Calamity? A tribe took them in and they then took a tribal abbreviation? If you do the Amal'jaa quests, the Miqo is a Sunnie, but doesn't have a tribal letter (She's from the U tribe). She left her clan to join the Brotherhood of Ash, and so she dropped her U tribe letter.