I'm not sure about that one...I had a similar situation recently as well. First I was looking for a reservation for Chef Mickeys - nothing. Then one opened up and I grabbed it. It wasn't the best time, but I took it. I kept looking online at Disneys dinign reservation and nothing. I checked dinner and breakfast for almost every day we're going to be there. Then one morning I went online and viola! There was a better time for the day I already had a ressie and it seemed like a bunch more opened up....I changed my ressie to the time I wanted. I then checked again a day or so later and no ressies available for Chef Mickey.
I was thinking, as you, either a bunch of people cancelled their ressies (maybe those who had 2 holding and decided which one to use) or Disney holds on to some ressies and then releases them at different times. I really don't know and am a bit curious, but mostly am happy that I got the ressies I wanted.