I believe the "weekend" rate is supposed to begin at noon Thursday and I believe requires a Saturday keep, so the ORD station might very well have run the contract at the "weekday" rate.

So perhaps the Hertz computer muffed it, but if it spit out a confirmation at that rate, Hertz is bound to honor it (especially if a corporate-owned site, which I imagine ORD would be).

Or it's some special promotion they're running at ORD. At which point it shouldn't matter what the pick-up time was.