Longer ferments eat more sugar. That is most of the answer, but the oak does something too.

A classic runs seven to nine days. Reserve runs twenty-one to twenty-eight. By the end there is roughly a third of the residual sugar left, which is why it reads as dry rather than tart-sweet.
The barrels we use came out of a cider house, so they contribute very little vanilla and quite a lot of grip. If you like a dry cider you will get on with Reserve immediately.