You probably thought that because the yellow and the purple are both 100% saturated (in HSV) that the intermediate would be 100% too. But no - intermediate shades between to changing hues become less saturated. Explaining why is difficult, maybe somebody else can.
I'm not sure, but I'm pretty sure it's because Gray is the halfway point between complements. In your first post, your colors are moving around the color wheel:

A more extreme example would be to travel through the center first:

But your colors aren't perfect complements, so that wouldn't be optimal. The gradient would travel through the color wheel:

Which means your colors should be somewhere around here. I also edited the AA:
