How are new ratings calculated?

I'm in a tournament. I see some unfamiliar 2200. "Ooh," I wonder, "Better watch out for this guy."
But if I click on his name, I see that he joined 6 hours ago. He's played like 5 Blitz games.
We're playing Bughouse! His Blitz rating isn't even 2200!
Almost without fail, he loses vs. a 1500, and his rating drops several hundred points, His partner is annoyed that he was a "fake 2200".
Question: How does chesscom "calculate" ratings for players who haven't played that variant yet?

Ratings should default to the initial account rating, in most cases.

https://support.chess.com/article/671-how-do-i-create-an-account

Normally if they have a high rating and joined in the last 6 hours there is a good chance they are cheating.

But as far as I know there isn't any bug house engines so he's probably fine.

Early rating your k-factor is every high so you can up and down a lot when you just start playing.

Once you hit 25 or 30 games things settle down.

As I've looked at it, your initial rating is whatever you say it is. So if you assess yourself at 1600, that's what you start as.
The weird, interesting thing is if you go play a variant. Let's say you're 2250 at Blitz or Rapid. But you've never played Bughouse. Well, chesscom says your initial Bughouse rating is 2250. Butt since you don't know the first thing about the game—sacs, defense, communication, the clock—you and your pard lose to a 1500 pair. Your partner is frustrated because you were a "Fake 2200."
I don't know how best to handle ratings. The system has lots of anomalies, though.

It's the K-factor. Basically a new player gains/loses tons of points in the first few games and it eventually balances out. But if you're lucky and and win 5 games in a row, your rating will seem insanely high.