Put simply, just because you can see seats available in a fare bucket, doesn't mean there is a fare available for you to buy from that bucket for the route you are travelling. For a fare to be available, not only do you need seats in that fare bucket to be available for sale, but you also have to meet certain conditions of that fare type, such as advance purchase which may be e.g. 28 days prior to travel on an R class fare (hypothetically). However, a passenger buying a ticket from PRG to JFK may be able to get an R class fare because tickets for that routing may only have an e.g. 14 day advance purchase in R class (again, hypothetically). Hope that clarifies.