It is fed from game data, specifically the “available list” sent at login time from the game server to the client. Presumably, this list is what powers the rotating teaser troop at the old chest interface (is that still a thing?). There’s no way of telling if that list is the same list that actually is used at pull time, nor if the stated odds of rarity per key are accurate, as that logic happens purely server-side, so both @Taransworld and I have to make some guesses.