From what I understand the plat overide is under Guaranteed room availability benefit. And the T&C for this benefit is:
Applicable to one standard room booked by 3 p.m. local time 72+ hours before day of arrival at participating hotels. Member must pay regular RACK rate. Black out dates apply. Not available at resorts. Room upgrade and Free Night Award redemption do not apply when this benefit is utilized. Minimum length-of-stay requirement must be met.
Which is why OP needs to pay a higher price for the room.