We went around the same time last year and got most of what we wanted at our 180+ day. The weeks after that, very little was left.
So I have a theory.
On our way back to the Poly on the bus, we passed by the MK parking lot, and it was nearly empty! Yet there were still plenty of people in the park. I think that, in general, people driving to the MK for a day will not do sit downs and ADRs and will most likely do quick service instead.
It's the people staying at the resorts (both on and off Disney property) that do the ADRs. And during the early December time, pretty much all the resorts are near full. And all of these people need to eat and are most likely the people who take the time to make ADR's. It's also a great time for DVC members to stay because it's not peak or holiday season, meaning less points to use.
So I would say that the majority of people you see at Disney during this time are actually staying there. And these are the people who make ADR's. So in the end, ADR's are just as hard to get at this time, than at peak times.