I am not entirely sure, but my understanding is that GS indeed is somehow random, and then I mean the fact it increases or decreases.
Unless you player plays to few, or to much minutes in a weak, the game shape will balance around strong. so respectable, strong and proficient are normal GS at any time during a season for players getting the good amount of minutes.
If they get to few, or to many weeks in a row, it will slowly drop, and the longer it lasts, the lower the GS will get. Once they get the right amount of minutes again, they'll start going up.
I don't think there is any way in which you can control to have a player stay strong or proficient all the time. But I've never seen a player going to average if he did play enough minutes weeks in a row.
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