who told you this, someone who has a vested interest in a dyno dynamics set of rollers i presume?

as above, you definitely should get a higher reading with a hub dyno than a chassis dyno as you don't have the losses of the tyre/roller contact, which is not unreasonable to estimate at around 70hp - in fact you could check it by putting the same vehicle under the same conditions on both a hub dyno and a chassis dyno and it will tell you.