I phoned a hotel manager I know to ask him about what happened to me. He used to work in a IHG hotel. He told me that IHG compensates the property at about 20 bucks a night for award rooms unless they get to 95% occupancy. If it's above that, they give them considerably more. This sounds right, because I've heard that SPG uses a similar formula.

It's really the individual hotel that sets how many rooms they want to offer. I guess 20 bucks isn't enough incentive for this hotel to offer more than 3 rooms. That's a shame, as it's a stupid inconvenience for me, and just means that some hotel rooms are going to go empty that night.