The new homes being offered is rather easily explained as the builders finish up the building process, the homes come on line for sale...if not even sooner (as they are being built). In terms of the resale offerings, it might just be a timing issue. I looked over the homes for sale in Riverwood, and found 48 total listings. Noticing this is a planned community of around 4000 acres, that does not seem like a particularily large number.
Good luck in finding the "perfect" home for you and your family!