Air temperature can hugely affect roller results as well...on a hot day the fan running indoors does fcuk all expect force warm air through the engine..if the car feels quick and is boosting ok thats all that matters...people get carried away with rolling road figures when half the time they aren't even accurate.
People are trying to reverse engineer flywheel figures that manufactures get from bench dynoing the engine....so many variables that can change ..tyre pressure, air temperature, air flow, transmission losses...some operators even use different gears...if it was a scientific process it would never stand up to scrutiny...the Yanks use power at the wheels because it is the real reading the rollers provide..the flywheel power is an estimation.