Drgwen says:
... Weird to me that Wolf isn't Wild, and Tainted is, but ...shrug.
...
Morphling says:
Yeah, I actually didn't even realize Wolf wasn't Wild until I was like a third of the way into this game, and when I did it confused me.
The Circle divisions have always been a bit weird. I think 2e defined them pretty well, though.
Mortalis is people, ones that don't fit the other Circles but are still in the know.
Power is people who have sought out and gained power, so 'wizards' (including Oracles, even though some Oracles may have had that power foisted upon them rather then seeking it out).
Wild are alien, 'not from here'. Fae from faerie-land and demons from 'hell' and (though I can't check right now), presumably, Angels from heaven? They were never human and don't understand us nor are understandable by us.
Night were people, but something happened and they were changed. Many consider the whole Circle as 'undead' (werewolves are often clumped into that category). While they are changed, they are still much more human and remember what it was like to be human.
The Tainted is, indeed, a strange one. I could see them as Night, but the book is saying they are more demon than human.
Persephone has repeatedly said she is a demon, now. The members of Night still have basic control over their own actions, the Tainted is driven by an otherworldly contract, they are not meant to be understood or predicted?