Uh... just a small question. How are the Tactics Trainer problems found? Please answer.

PS. I just realized the title makes it sound like  Tactic trainers has a error (which I detected). Well, it doesn't (or at least, nothing which I detected).

when you go to the main menu on top of the screen you click on learn.

the third option is tactics trainer.  

Most of the tactics trainer problems positions come from actual Chess.com games and master level games from old game databases.

Does anyone know how the difficulty or points are assigned?  As I go higher the difficulty vs. problem rating sometimes seems confusing.