IMO The fatigue increase changes the RESULTS on the pitchers cards, it doesn't change the die roll to go to the hitters card.
Back in the day of collecting stats, downloaded seasons worth of the play by play boxscores... I did find that as fatigue rating decreased from f9 to lower levels the % of times rolled on the batter's card increased. As I recall, it went from very close to 50% at f9, up to about 60% when pitchers reached f2, f1 and f0.
That is not to say that the computer logic didn't also influence which rolls on the pitcher card, say avoiding K's and DP's and rolling more on hits and walks... but at least one, if not the main way, that the computer dealt with pitcher fatigue was to favor the hitters card in such a manner.
At one point I had also wanted to test another hypothesis... that the computer yielded way more than the typical amount of grandslams when the bases were loaded, but that fell onto the large not-yet-done list.