When I attempted to book 93 I did the same thing and it showed booked immediatly. When I called and talked to a real live person they were able to confirm that it was still available even though it showed booked. It was explained that if interest is shown in a particular site it is immediatly "booked" to place it on hold for you. It will remain in place for several hours then manually reset by reservation staff if no $$$ has been put down. This protects the 1st come first served policy. If you have to go get another credit card ,or you mis-key, or your computer takes a time out ,your choice is protected. Several of the booked can resurface as available if no deposit has been actually placed on them.
A bit confusing at first but it is very fair policy.