Let Me Know if You Have Allison Transmission Fluid Questions

I read the whole thread and if I missed this, I apologize.

Let's say there is a drain interval of 150,000 miles or 48 months, whichever comes first. Or 50,000/48, whatever.

And let's say in 48 months the vehicle is actually only driven 20,000 miles. I understand that sampling and testing can be employed to extend the drain interval. But...

What is the reasoning behind specifying a calendar time? What happens to the fluid as it sits in the transmission in a low mileage situation? ie How does it go bad?

Thanks!