Ask HN: [Gamedevs only] What hidden RNG modifications have you seen in games?

I mean situation when to players it is said that something happens truly random but in fact, it is made so that it maximizes in-game purchases and is not truly random. For example, it is determined (data mining and etc.) that some RNG modifications are leading to more in-game purchases if RNG works not as true RNG and RNG system is accordingly modified, but to players it is presented as true RNG system.