Notice that there is only one QCMA within each of the impact basins Crisium, Nectaris, Humorum, Serenitatis, and the inner part of Imbrium.
Surprisingly, QCMAs have both mass excesses and deficiencies.
Evans's group explains QCMAs with negative gravity anomalies as marking craters that formed into early mare lava flows, excavating through the lavas into the underlying lower-density highlands material.
It's surprising that large QCMAs occur in Mare Frigoris and Oceanus Procellarum where no underlying impact basins are thought to exist, and thus the mare thickness might be less than needed to completely cover crater rims.
The detection of QCMAs is exactly what would be expected, and like most good discoveries raises more questions for investigation.