The funny thing, is that it works both ways.
For example, a driver that runs 1:30 laps, runs those laps because at some point there's a ... (for lack of a better word) flaw, in there driving style. So, let's say they constantly overdrive the corner on entry. This tuner, will tune the car to accommodate the fact that they overdrive on entry, without even knowing they do it. To them it's just normal, and they assume the car just has an understeer issue on corner entry. Regardless of how they tune the 'self created' flaw out of the tune, the point is, their car will likely enter the corner, better than a faster driver's tune.
The driver/tuner who runs 1:28, runs faster because they enter properly, less sharply, or just on the right mark/apex/whatever. They aren't accommodating a less than optimal entry, and it allows them to gain an advantage in exit ability, for a faster overall corner.
So, a 1:30 driver, gets into a 1:28 tuners car, and all they feel, is that the car has this horrible understeer tendency when they enter the corner. It's easy to say 'Well I can run 1:28's so the car is fine, you're driving it wrong' but that's not how tuning works, and it's why a faster driver doesn't automatically mean faster tunes.