Some of them make random errors to simulate a human. Others limit the number of moves they look ahead to do it. None of them play like a human, but that is the state of the art as of now for faking it.

The best approach would be to train the AI program by playing against actual humans of the given strength level being simulated, but that would be impractical as it would take too long and the players being used would likely change rating (hopefully upwards as they learn too) as they play.