I haven't seen one in awhile, but I recall many ads that had rates quoted as per person per night. And it always had the note below it saying "Based on double occupancy" which were weasel words effectively saying it was same rate single or double.
Look at the ad again and I'm sure you'll find that statement somewhere, even if it's in the T&T's. If by some longshot it isn't there anywhere, I'd press the issue and get the pp rate.