Uhh...the Prius C uses the 1NZ-FXE, exactly the same powertrain as the 2nd gen Prius but with worse aerodynamics. If I am not mistaken about my Prius generations, the 3rd gen uses a 2ZR-FXE which is a bit more efficient.
The 2ZR has lower valvetrain friction than the 1NZ, dual VVT-i, and they were probably more aggressive with the cooled EGR flow. It's only ~4% more efficient though.
The only reasonable explanation is that your car was simply consuming more power. Maybe there was a bit of a headwind. As you can see from the BSFC chart that was posted, the mid 2000rpm range is where the efficiency peak occurs.
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