On my last trip through the US (don't remember which airport...) the TDC was filtering between PreCheck and Premium and handing out PreCheck advertising cards to the people who didn't clear PreCheck. He was saying that people cleared for PreCheck had a 10% chance on each trip to be selected for normal screening.
Of course, he also didn't believe me when I said that I wasn't able to get PreCheck (not having a FF membership of a US airline in the program or being eligible for Global Entry).