In my opinion, I still think it works like this:

You have a roll of the 1-100 RNG against a 10% chance of Atma dropping on every FATE you're registered as participating in. (Maybe less if it's not a Gold/Success.)

That is both high enough for people to get all 12 quickly, and low enough for people to have rotten luck and not be done after upwards of 60 hours of it. It's also high enough for people to mistakenly see patterns where there are none.