Here are a few other considerations concerning "gravimetric scans". Wasn't it Pierre who showed the world his scam... sp. scans, as evidence for internal ramps.
So, either they tend to prove little or nothing, and/or they are subject to misinterpretations.
Another possibility is, the AE choose denser stones for the inner core, resisting internal weight/pressures, and less dense stone for the peripheral. The quarry stones are not all the same density, and may have change from one location, to the next.
How this work out?