I've seen this for years, and at all levels of resorts at Disney, and for different types of discounts. (I.e., if you look for x number of days - usually less than a week - no availability or not for your preferred category. Then, if you add a day or two or more to the start or end of the original search, all of a sudden there's availability - even during those days for which it previously said no availability).
Disney must have an algorithm that predicts longer stays are less likely to cancel, or are easier to schedule, I don't know, but this is very common in my experience.