U.S. octane requirement question

[CIK7]

Outside the US, a different octane rating is used. For my post I'm using the US numbers.

In the manual (pic below), it shows the NA is higher compression (10.5:1) and turbo is lower compression (8.5-9.0:1). I already knew that. My understanding is that a lower compression engine can use regular 87. The higher needs 91. But either Volvo wants us to use 91 or someone has trained us to think we need to use 91 on the lower compression engines. What is it that I am missing?

[abscate]

Compression ratio is the static ratio of chamber with piston down and piston up. When a turbo boosts, it pushes extra air into the cylinder and the effective pressure in the cylinder is higher? How much? I’m not sure.

I run either 91 or 94 in my turbo cars and 91 in my NA cars