Yes that makes some sense I suppose if they could be confident that they will surely sell enough 2-night stays. In the end I booked the 2 nights online, and called the front desk (note front desk not reservations) on day of arrival, and asked them to change to a 1 night stay and they said it was fine, and changed it without the slightest of hesitation. As for using the Diamond guaranttee that is usually only avail on wrack rates? I know that some of Hiltons corp rates apply even with "last room availability."