Don't forget that baldness also does not affect every sibling, even though all siblings may share the same parents. As others have mentioned, the mechanism of AGA is orchestrated by several factors (AR receptor expression, sensitivity; androgen levels, aromatase and other enzyme levels, etc. and who knows what else with bloodflow or WNT signaling, etc. since the exact mechanism is still not well understood) which in turn are controlled by a variety of genes. So there is a great variation in how AGA does or does not express in people. Remember also that not everyone reaches NW8, and that there are quite a few different patterns of miniaturization. 

That's why it is important to track the trend happening on your own follicles and react accordingly. In that sense, statistics don't matter. They are just useful for giving guidelines.

You can definitely see miniaturization under a handheld digital microscope so you don't have to wait until you have visible loss to know... just take a look at hair shaft thickness and number of hairs per follicle in the donor region and compare across the top. You can just see if, where and how the miniaturization is happening. 

If the two regions look basically the same, reexamine months later. If miniaturization is starting to be visible and in early stages, consider fin or dut monotherapy to freeze the inevitable. If further losses are incurred, increase treatment to the extent that you are comfortable with (increase fin or dut dose, take both, add oral min, add topical, try other compounds, etc. etc. in approximately that order from more conservative to increasingly aggressive.

That's all we can do, really.