I noted recently that a 4 person itinerary that I'd reserved went from $330/head to $370/head. Both fares were from the "X" bucket. A closer check reveals that with the exact same dates/flights but booking a single person at a time I could get 4 seats at the lower price.

When I've seen this kind of thing before the fare buckets were different and there were only one or two seats left in the cheap bucket. Here they're the same! Unfortunately I can't see the actual X inventory since the booking engines I've looked at don't show all 14 CO coach class buckets.