FtWildernessGuy is right, the occupancy rate is going to ebb and flow, even during busy season. There are always last minute cancellations, perhaps there is a reason Disney wasn't filling those specific sites, etc... I'm sure Disney keeps quite a few sites empty throughout the Fort to handle any kind of contigencies.
But the chances of having every site filled while you're on an extended trip (more than just a weekend), even during peak season, isn't as great as you think it is. Most people do have problems getting reservations during peak times, but that's normally because their reservations cross over the weekends, and even though it might be more open during the weekdays, the weekends most likely will be booked.
But the chances of having every site filled while you're on an extended trip (more than just a weekend), even during peak season, isn't as great as you think it is. Most people do have problems getting reservations during peak times, but that's normally because their reservations cross over the weekends, and even though it might be more open during the weekdays, the weekends most likely will be booked.