I believe the higher numbers work like this: every 21 turns, Threes generates a number from 0-20. After this number of turns elapses, if the highest number seen is 48 or greater, Threes skips drawing from the normal set of ones, twos, and threes, and instead generates a "special spawn". The special spawn is a number between 6 and <highest number seen> / 8 (I believe the distribution is uniform).
Source: Threes writes a preference plist that allows it to resume games if the app is killed in the background. The keys to look at are "PseudoSpawnSpecialIndex" and "PseudoSpawnSpecialList".
I believe the higher numbers work like this: every 21 turns, Threes generates a number from 0-20. After this number of turns elapses, if the highest number seen is 48 or greater, Threes skips drawing from the normal set of ones, twos, and threes, and instead generates a "special spawn". The special spawn is a number between 6 and <highest number seen> / 8 (I believe the distribution is uniform).
Source: Threes writes a preference plist that allows it to resume games if the app is killed in the background. The keys to look at are "PseudoSpawnSpecialIndex" and "PseudoSpawnSpecialList".