For each resort, they sell the amount of points that it would take to fill the rooms nightly for 51 weeks of the year (i.e., the DVC inventory of rooms which is about 96% of each resort). That is legally not oversold. However, there are several factors that can make it seem that they are oversold, e.g.:
1. There are a number of times of year when demand exceeds available rooms such as any holidays, much of December when you have low point cost and all the Christmas decorations, food and wine time, most spring break weeks.
2. The weekday/weekend dilemma: weekdays (Sun through Thurs night) cost significantly less than weekends. Vast numbers take advantage of that by going only weekdays. Consider someone who has enough points for a week but instead of going for 7 consecutive nights that includes a Fri and Sat, they can get 9 nights a year by just using points for weekdays; result when large numbers are doing that: demand for weekdays significantly exceeds availability most times of year. It is noteworthy that Fri and Sat nights can often be found available most times of year even at a few months out when it is otherwise impossible to get weekdays.
3. The larger resort v smaller resort 7 month issue. SSR and OKW are the largest resorts, combined they have close to or more than 50% of all owners. Also, neither is a "close-to-a park" resort. When it was just OKW, there was never really much of an issue of getting overwhelming demand from OKW owners at 7 months for rooms at BWV, VWL and BCV. The likely explanation: most who owned at OKW preferred staying at OKW every time they went because, among other reasons, it has larger rooms and significantly lower point costs. SSR is lower point cost but not a great difference. Room sizes are the same as BWV, BCV, and SSR. The 7 month demand for BWV, BCV and VWL has greatly increased with the addition of SSR and thus it has become difficult to find rooms at those three many times of year at less than 7 months out. BCV and VWL have the added issue of being very small resorts in comparison to SSR.
4. The smaller contract issue: Originally, when it was just OKW a new buyer had to purchase a large number of points (Can't remember actual number but it was well into the 200s). That was steadily lowered over the years although it is now "up" to 160 (but AKV even had a 100 point deal). In the last eight years, internet sales have grown from almost nothing to a major resale market, and that has resulted in a large small contract market where there are large numbers of "add-on" contracts of less than 100 points being sold. The result: there are now a lot of owners with a low number of points (for example under 150 each). Thus, you have far more owners (even though with the same amount of total points) competing for those high demand times of year and those Sun to Thurs night stays. In other words, you just have far more owners competing for the times that were already high demand.
5. The home resort safety reservation: Have an owner not get a ressie at his home resort and he quickly learns his lesson -- book at 11 months out. Much larger percentage are doing that now than in earlier years. Also, larger percentage are booking home resort now at 11 months even when they want another resort at 7. The more that do those things, the harder it becomes for even owners to get high demand times of year at 8 to 9 months out and they have to start waiting until "switch" time right at 7 months to get their own resort, causing more of them to start making all their ressies at 11 months out. In other words, the more that report having a problem the more members who will feel that they must reserve at 11 months, thus adding to the problem.