Is this unique to IC?
Or can you similar find better rates at an Holiday Inn site for HI compared to the PC site, or at an HI Express site for HIX compared to the PC site, or at a StayBridge site for SB compared to the PC site?
(So far, I've only ever bothered to check the PC site, and when I don't like the rate I either use points or just move to another hotel family. I didn't realize that the brand's own site could possibly be lower than the PC site!)