I think the reason it dynos better is because it enclosed and not getting the hear soak. When on the road the induction kits have a larger surface area on the filter and with the airflow while moving it while be able to swallow more air where as the airbox will have a smaller free area so it doesn't necessarily mean that when moving your gonna have that extra gain over the induction kit.
I've bought one of these gadgets that plugs into your ecu and monitors all your actual speeds, acceleration, g-force and an actual on the road 0-60 etc so when it arrives I'll post up my results
On the road my intake temps were lower with a pop charger than a std 03 airbox & were lower again with the ARC box.
But when dynoed I was told by the operator that design of of the popcharger was losing me power. On the same rolling road a few months later (only changes were headers, stock intake & removal of plenum spacer) I gained 13hp over the figures with a popcharger & spacer