Recently - within the last few weeks, when I fly out, TSA has not only asked for my ID and boarding pass/security document but also asking and verifying my flight number. Not sure if this is the case for others.
The only times I've had that happen are when I had been checked in for multiple flights at different times. For instance I started the day checked in for an ATL-SEA flight but as it got closer to time saw that wasn't a good option so I took myself off of that flight and then checked in for ATL-DTW-SEA. Even though I had deleted my original reservation, it still shows up in tsa's system. So in that case they have to ask which one so they can verify. There may be other reasons that happened but I've had that happen to me three or four times and that's always the reason.