Not really. The car is a FWD car. So when the rear wheels aren't being spun up, the car should produce ever so slightly more measurable power instead of less.

Not really. The car is a FWD car. So when the rear wheels aren't being spun up, the car should produce ever so slightly more measurable power instead of less. Because, as you stated, it didn't have to spin up the rear wheels, etc. The dyno can tell if both wheels are being driven or just the front ones. If it couldn't, then the results wouldn't be off by 10% but they'd be off by 50% as the dyno wouldn't understand that by having the two drums linked they got twice as heavy and it'd do the calculations based off of the inertia of a single drum instead of both drums.