I think what you're observing depends on how they choose to display limited data. I would not expect that simply rolling off the batter's card is enough of a penalty to make a difference. Sure, there are more hits on batter's cards, but not so much more to simulate pitcher fatigue.
There would indeed be an observed skew if they chose to display the result from the batter's card when that result was selected per my hypothesis above. If this is the case then at F8 there would be a .475/.525 ratio of pitchers cars to batter cards and dropping with each step.
Edit:
It occurred to me over dinner that we can discover whether its one way or the other. If the algorithm simply shifts the number of rolls to the batter's card (say from 50/50 to 60/40) then the percentage hits/walks per batter's roll will remain constant at all fatigue levels, but if it the penalty occurs like I hypothesized then the percentage hits/walks per batter's roll will increase with each drop in fatigue rating.
For example assume (I don't know what the average values are) that pitchers cars get batters out 66% of the time but batter's cards get on base safely 40% of the time then a batter is safe...
F9 = (.5 * .34) + (.5 * .4) = .370
--> batter's cards 50% of results shown with 40% of those safe
F8 = (.475 * .34) + (.025 * 1.0) + (.5 * .4) = .387
--> batter's cards 52.5% of results shown with 42.9% of those safe
F7 = (.45 * .34) + (.05 * 1.0) + (.5 * .4) = .403
--> batter's cards 55% of results shown with 45.5% of those safe
F0 = (.275 * .34) + (.225 * 1.0) + (.5 * .4) = .519
--> batter's cards 72.5% of results shown with 58.6% of those safe
F0-10 (ten steps below F0)
= (.025 * .34) + (.475 * 1.0) + (.5 * .4) = .684
--> batter's cards 97.5% of results shown with 69.2% of those safe
Since the percentage safe/out varies with each pitcher/hitter combo, I would think that a fairly large sample size would be needed to detect the effect.