Just guessing here - but it's probably simple "Marketing". Tuner companies want to entice you with bigger numbers than their competitors. As most know, Horsepower is just a calculated number = Torque X RPM / 5252. So by simply spinning the motor faster (but no change in Torque) - you end up with a proportionally larger HP number. You won't go any faster, you won't get better lap times - but you WILL wear out the valve train and other mechanical components much faster, and (taken to the extreme), blow up sooner.
Auto manufacturers very carefully set RPM limits for a variety of reasons, mostly longevity. And believe me - their Marketing people want the biggest HP number possible, too - but they settle on some reasonable compromise with the Engineering department. Your Tuner doesn't care as much, needs to show you a more impressive dyno sheet, and figures there's enough safety margin in the OEM number to keep you from blowing up before exiting their property. So move the REV limiter up 500 RPM (or the really irresponsible might eliminate the limiter entirely), and send the wide-eyed gullible customer out the door with his freshly printed dyno sheet to share with his buddies at the tavern. Those in the "know" will look at the Torque curves, not Horsepower, when making before & after comparisons.
So - unless you know the seller really well, be "wary" of buying a used car with an aggressive "Tune" - especially one with no rev limiter! The money you save could be used toward buying a car that was originally & properly engineered with "MORE Horsepower" in the first place. Perhaps the Tuners monitoring this list will disagree??