And, expanding on what Christep said, they do the same with awards and discounted premium seats. A few years ago, before the great travel slump, they hardly allocated any seats at all to the A bucket on 830, 831, 840 and 841. Thus it was impossible to get these flights on an AONEx ticket. Now, however, they're plentiful.

Also, if you watch for F awards, you'll see that availability comes and goes as they try to manage revenue pax versus awards.