Yeah, it is inaccurate, but then, if they're going to test cars in different weather conditions (dry, mildly moist, wet, monsoon etc) and just compensate for that by saying "We'll take 4 seconds off that time because it was wet", even when Webber was saying there was no way he lost 4 seconds, then the test is hardly accurate to start with.
I don't think many people really think the laptimes are valid for serious comparison, it's just fun to watch the cars being driven hard round the test track.