The largest single item in availability is the demand for the areas in question during that time. So yes it will be easier to book the more difficult options during slower times. Likely as easy or easier to book those options during slow times than the easier options during peak times.
Often there are threads around here that go like this:
Q: I want to get a Boardwalk Villa Standard View Studio the first two weeks in December at seven months, what are my chances?
(Note, hard room, hard resort (view), hard time)
A: You won't have any problem, I managed to book a standard view one bedroom seven months out for early Feburary last year.
(note, less popular room, same resort, slower time. Occationally, the post will even come back that they got a preferred view - which is easier to get)
I think what Dean is saying is that response to that question is pretty meaningless.
Likewise, the following exchange will happen:
Q: I want to book a BCV one bedroom at eleven months for late January, should I call day by day.
And someone is sure to answer:
A: BCV is a very popular resort, I'd call day by day for sure!
(Chances are really good you could call seven months out and a one bedroom at the BCV would be available in late January).
And even the "easy to book" SSR doesn't have rooms available for nights in October and December already - but you can probably find something for late August at the "little three."