It might be okay.

Going 0.4 mph faster on say 22 more watts is not crazy. If you were to assume a speed of 22 mph then to go 22.4 mph would require about 11 more watts (assuming flat ground, no wind, .31 cda, 85 kg).

As already mentioned, Quarq is reading before the drive train losses. That could be a few watts.

Then of course power meters are inherently built with 1-2% error. At 220 watts, that could be a few watts as well. So all added up, you may have 16-20 watts accounted for.