Frank will tell you if you ask him. He started off at, iirc, 500 miles. As oils advanced in longevity, he moved the mileage up. People are hard to please. Some want it over with quickly. More think it uses too much oil for the process. After testing it out, he came up with (basically) 3k/3k ..and go about your business. Since many are used to 3k (there abouts) this satisfied the most people in terms of seamless usability. For those wanting more immediate results, he tested double doses for shorter durations and found it acceptable for getting many of the benefits without the protracted process.
Again, Frank doesn't know the condition of your engine. So there's an uncertainty factor that has to be weighed in the mix where you want a long enough duration to assure adequate dwell time, while not going too long with a filter that may be plugged. If the owners knows that he's got a heavy deposit condition, the instructions state that an intermediate filter change is recommended.
He's explored many scenarios of time, mileage, and dosage and configured the instructions for the broadest customer base while providing the most satisfaction in performance.
What he doesn't want is anybody doing anything (we're talking inexperienced ..or first time user) ..not getting what they expected and complaining that it didn't work as promised.
Sprintman has been playing with this stuff for years on a wide variety of applications. His experience has led him to taking the latitudes he does with the product. This is a big departure from Frank saying "Go ahead and freelance Auto-Rx into your own image of how it should work. I can't tell you how to use the stuff ..it's all in your hands."
People need simplicity and Frank has bent over backwards to try and streamline this into an easy package, but no matter how he fashions it, there are still legions of people with niche circumstances and exceptional conditions.
It's a challenge to configure a "one size fits all" in a nation of individuals that only consult the instructions when all else fails (raises guilty hand)