If you look at your graphs, you can see that both of them have a "Quality of focus" divided by two on the side. The visual difference on those graphs is due to the fact that the vertcial axis in the first graph starts as 0. Therefore, your graph does not prove anything.
It would be meaningful to have crops so we can appreciate the difference.
There might be differences, but the Nikon AF Fine Tune units scale with the largest aperture of the lens. If you have time, try a 85mm at f1.8 and add 20 to your usual AFFT and measure how the focus moves. Do the same with an f4 lens. You will see clearly the focus moving further away now. Therefore, the blur should be about the same.
The scaling might not be perfect, but your graphs show it is: quality of focus is divided by 2 at the same distance from the pick.