Our reservation system does not remove the site from inventory until full payment is received.
Therefore if you select a site, there may be a chance that someone else is in the process of finalizing their reservation for the same site and when you enter your payment information and select checkout, the site may be no longer available.
If we were to pull the site from inventory as soon as you put the item in your cart, everyone would simply place items in their cart hours before they are able to book simply so that no one else could book it. As well we might have people who open several windows and put multiple items into carts while they try to decide which campsite they may want. It only takes four or five people to do this and when someone goes on to look for what sites are available, all site would be showing red as if they are reserved because they are being held by someone.