[Silverstone]

"Official" power measurements are made on an engine dynamometer (brake) with a direct link from the flywheel to the dyno. The measurement is corrected for atmospheric conditions (temp and pressure) to give a standardised number. The SAE, DIN and ISO standards are all slightly different!

A rolling road (chassis dyno) can only give an approximate measure of flywheel power, since the losses in the transmission and tyres are rarely known exactly. They give an accurate measure of power at the wheels.

Since power at the wheels is what actually matters in the real world, a rolling road is a useful piece of kit. Nobody is quoting power at the wheels yet, so we can't compare.