Roger, I see the same thing with my Kickr Core and my PowerTap C1. The C1 consistently reports 10W higher than the Core. I also have a Stages (left only) which reports more or less the same offset as the C1. I contacted Wahoo support about the differences but they kept coming back to the difference being drivetrain loss and +/- tolerances. I might agree with them on that but not when the 10W difference is the same at 100W as it is 400W.