It's for the added weight of accessories that were installed by the distributor or dealer, before the vehicle was delivered to the customer. The vehicle was manufactured at x,xxx lbs, but was delivered at x,xxx + 11 lbs. The vehicle's load capacity must be reduced by 11 lbs to maintain certification. Accurate tire loading and capacity labels appear to be a big thing, as there have been several recalls to correct these across various makes and models (none for Prius, AFAIK). Your 11 lbs are likely the floor mats/cargo mat.
