When I say accurate, it is as accurate as I need it. It is not as if I record both instantaneous readings and then compare them. I look first at one, then at the the other so it's as accurate as that. I also travel at a constant speed in an open location so minimise errors from buildings etc.
The ScanGauge is more accurate, as every top up you add a correction to the error which you don't do with the on-board. But to be pedantic, it is only more accurate, not completely accurate. Of course, neither is a computed figure, gleaned from one tank of fuel. The figure beside my avatar is a computed figure from (currently) 56 fill ups, so that is pretty accurate.