Read it, it's very presumptuous on thier part to assume wheels spin on all rollers, some like the one I use regularly re grip the rollers, and calibrate it often, by measuring run down losses flywheel bhp comes about, as well as crank, the difference between the two is the trans loss.
Some rollers can't be beleived anyway, as the trans losses are sent to them on a compy link or data link, that's why they are easy to fiddle.
The ones I use also have a engine dyno, and the rollers read within 1bhp of the engine dyno, so good enough for me.
Trans losses also increase with speed, pumping up tyres on a decent dyno gives no more power as trans loss, ie costa down puts it back to square one accuracy.
tabetha