There are "random" checks as well - where you get a line of asterisks on your boarding pass, and get pulled aside at every step for extra checking. But if you're on a watchlist, it will happen every time, not 1 in 30 or so.
I put "random" into quotes, because the only time it has happened to me was when I was standing in line behind a middle-eastern looking woman in October 2001, and they seemed to pick her out by racial profiling. Then I was asked if we were together and I must have said "no" in a suspicious tone of voice or something, as they decided I needed to go along with her to a separate checkin queue for extra checking. It was clearly decided by a human, not the computer who was going into this queue, but the system may have changed after that. Booking one way flights and having someone else pay for them on a company credit card reportedly increases your chances as well.