https://en.wikipedia.org/wiki/Mach_bands

Our eyes aren't particularly sharp, so our brains compensate with what is essentially an unsharp mask filter to better detect edges. The artifacts from that process is called Mach bands. My hypothesis is that this is really what the microcontrast people are seeing, not a property of the lens. A less sharp lens taking a picture of the same subject is simply more likely to produce features that trigger Mach bands. Lens makers can't control the two metrics even if they wanted to, as they are inversely proportional to each other.