Dyno's are affected by the diamiter of the wheels/tyres - just like a bike when you change gears on the front cog it affects what goes on at the back wheel - so the bigger the front cog the harder it is to rotate the rear wheel - if you got 19" rims on the dyno you'l get a different power reading to that of a car with 15" rims

a hub dyno gets around this by taking direct drive from the hubs, therfore not being affected by the rolling ratio's of the wheels.