I suspect it has something to do with the quality, or lack thereof, of various tools. My Klein and Wiha T8/IP8 drivers/bits fit snugly into every APX model I've disassembled (6000AN, 6000BN, 7000, 8000), whereas the same brands T9/IP9 are too big to even fit into the head of the screw. Contrast those with some of the less quality stuff I have laying around, such as Milwaukee impact driver bits, and the T9 will technically fit, but doesn't seat fully because it's still a little too big. The Milwaukee T8 fits a little bit loosely, which is also no bueno.
So I guess the answer to the OP's question (despite MSI providing the answer in every model's BSM) would be "It depends...do you have good tools that are precision made, or stuff that is sub-par and fits sloppily into machined screws?" If the former, T8/IP8, if the latter, I guess try a T9. 🤷‍♂️