Tafflyer

Don't forget the uses some of these buckets have apart from controlling prices. J and R classes for example are often permitted on feeder flights to long-haul flights in I class so that tighter availability on a short-haul sector does not prevent BA from selling a potentially way more expensive long-haul itinerary. This gets even more complex on code-share through bookings. AA for example booking in I class long haul will permit I or D buckets on the short-haul feeder. Airlines can control how much inventory is used this way and hold back vital inventory for their own use rather than a code-share partner. You don't mention which sector you are looking at, but I suspect it's a short-haul sector with a lot of connecting traffic, possibly also from code-share partners not necessarily in the JB.