Frankly that's just excuses they are told to tell you. They don't know any better than we do. The seats last year were visible for days and yet remained non-bookable on Alaska while being fully available on American. The seats I booked earlier this week failed to go through when attempting to switch from economy to business on the same flight, but then booked successfully when switching from a business seat on another flight back to this same flight.

And each time I booked, the inventory immediately was reflected on AS's website.