Not much at all - some people don't bother to assign seats so they just show up and get them at checkin.

I noticed that the frequency on your IAD-TPA routes increases on 17 Mar in the schedule, before decreasing on 1 May to support the west coast routes. I'm guessing that extra A319s are coming online early and they might as well get some flights out of them rather than parking on the ground for a few weeks.