SSR is (I believe) the largest DVC resort with 792 2-bedroom villas, of which 432 are lock-offs which can become 1-bedroom and studio villas. Many (not everyone) prefer to stay at other resorts and that includes a large number of SSR owners who change their reservations or wait to book at the 7-month mark to stay at a different DVC resort.
Given that, SSR is almost always the very last DVC resort to sell out. However, that doesn't mean that it's easy to book SSR at all times of the year or with little notice. You can use your online DVC account to check availability within the next 60 days and you will often see only scattered availability there.