My stab at an explanation is that the ssss is randomly generated and each time a boarding pass is printed it re-runs the ssss lotto. The first time the boarding pass was generated you were a "hit", the next time you were a "miss". The system could be more sophisticated and include some sort of algorithm (also know as profiling) that increases or decreases the chances of ssss based on a range of factors including frequent flyer status.

If my guess is correct, then this points to a flaw in the system since all a terrorist would need to do is ask for a repreint on the boarding pass when they see the ssss.