My SG Brcic was respectable last week with very low DMI, I was suspecting low sublevel as well. And the last game messed up my minutes, the whole week he only played 42 minutes. I was praying for it to at least stay at respectable. Then as I scrolled to see my players' gameshapes, he hit Strong.
I think there's always some kind of randomness involved, and I also believe those who state that it also takes into account the previous week's minutes (maybe the other previous week too) as well. Lately there's also been a discussion about Stamina affecting GS, it's still a theory, but some managers have sweet spots for certain players depending on their Stamina.