Depending on the base game's release date, this is perfectly normal and acceptable. If memory serves, a game's release date is around 2 months ahead of when it actually gets finished and ready for duplication, so as to give developers time in case they need it for major bug hunting. During that off time, it'd actually make sense to work on extra content for the game considering you've got all the game's developers right there, with the tools and assets right in front of them, thus saving time and, therefore, money.