Originally Posted by dkerr

Airfares and availability are very dynamic and complex. At some point in the past a R fare may have been offered that was eligible for a given flight. Now lets say such a ticket need to be changed, rules may require availability in the same class of service as originally purchased under the original rules, even if a fare is no longer available with the same rules. Thus you will often find flights close to departure time that appear to have many booking classes available, but you won't be able to buy a ticket at any of the "lower" classes.... they are "available" to accommodate changes that may be permitted by a ticket rule in the past.