The stamina decrease just doesn't seem to happen in some weeks, independent of minutes played.
I agreee with that, despite Virus answer. I have seen players at GS9 (and still got the data in my excel-sheet) who played a lot in one week, got no training and their DMI didn't change at all in the next week. But this is happening rarely.
As you will know, but didn't mention: There is a small random component in stamina training:
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