I don't think the inventory if incorrect from the website because if you book, they have an obligation to give you the room or risk forfeiting the Ultimate Reservation Guarantee.

They probably don't want to bother upgrading and dealing with a late check out, to save inventory for potential sale (especially if you are staying more than one night), or perhaps save on housekeeping fees (smaller rooms are easier to clean)?

I've definitely had front desk associates straight up and lie / say it's not available. Obviously at that point, they can't just backtrack once they're committed.