the thing is, all the shots by the SG were guarded by my PF, 22 shots all checked
Yes, that's because the two boxes are switched... or the manual text is wrong.
I already reported it to Marin several weeks ago. From my data, the manual is wrong about the two boxes.
It works like this (my opinion):
Inside box+1:
Best defender defends the best outside scorer and the box moves to the paint.
Outside box+1:
Best defender defends the best inside scorer and the box moves to the perimeter.
You played an Inside box+1. So for me, it's no suprise that your PF defended his SG. I have more such games listed in my fed. We made some tests this season.