Seats not available? Now seats ARE available!?

Hi, purchased a couple of tickets yesterday for the theatre through ticketmaster online, which is the first time I've done this. Clicked on ticket availability and there was only 1 ticket left in the stalls but quite a lot for dress circle. So, not a great view but I chose my seats and bought them. Just had a nosey now to see how many more tix had been sold and discovered 17 seats are now available in the stalls. I was so annoyed as this is one set price, no cheaper for dress circle seats. How does this happen? The seats they now have in the stalls are great and I would have so loved to get them there. I only bought the tix yesterday, how are these seats now available?