TSA can be inconsistent and random. This is partially by design and does not decrease credibility but rather enhances security. Unpredictability in security procedures is a key element of TSA's approach. They have also been open about this -- you'll find the following language at tsa.gov: "TSA will always incorporate random and unpredictable security measures throughout the airport."

For those who want to read more, I would recommend this WSJ op-ed by the former head of TSA. (Quote: "Predictability is deadly.")