Maybe it's because game shape can be random. For example, sometimes I've had players get about 70 minutes and still drop from proficient to respectable! (when not training game shape). Or play under 48 minutes and increase to strong or proficient... So it's possible that the player with 61 minutes would have dropped even more if you didn't train GS?
I don't train GS often, but the last time I had ok results. Half of my players with good minutes increased. & the rest stayed the same. It helped most for players with bad minutes it seemed... many kept the same GS, or at least didn't drop as much as they would have without training GS.
But I agree, it's best to avoid training GS normally. There's only a few extreme examples it makes sense to do it over other training types.