Testing can often paint a very misleading picture, not only for the enthusiast but for the teams as well.
During my years on the Formula 1 pitwall, you could never come back to the factory after a test feeling you really knew what had happened. If you were quick, you always knew the big boys would be able to pull something out of the bag. If you were slow, you knew you had to dig deep and find something, and you hoped that by the first race weekend things would look different.
There are always reasons for a team exceeding or falling short of its performance goals. Sometimes, it is because you had a problem that made it impossible to show your true potential, or because the car was not in its latest specification. But, in general, as long as you were honest with yourself you would have an idea of where you stood.