Thanks for this graph. It shows extremely valuable data.
However, before this discussion get too confusing, we must make a difference between resolution and contrast, and concentrate on perceived sharpness. There are two equations for the optimal pinhole diameter: one for max resolution and one for max contrast.
While a smaller hole will give more resolution (up to a point) a slightly larger hole will provide more contrast and perceived sharpness. The attached MTF shows that and verifies your statement.
Normally people subconsciously prefer contrast over resolution (high-resolution images are often too soft to be valued as sharp), and therefore, the optimum pinhole diameter based on contrast (and not resolution) is preferred.