Neither RAM prediction is based on possible maximums. PS5 comes from the GitHub leak, which if I recall correctly states a 256-bit bus at a maximum measured bandwidth of 531 GB/s. With that bus only 18gbps RAM modules would allow such a high real bandwidth, thus 576 GB/s theoretical bandwidth. The 16GB capacity is based on the assumption this is the minimum required for nextgen GPU and CPU. (It does also happen to be the maximum this setup can support.)
The XSX RAM prediction is based on
the render of the system shown by Microsoft at E3. That indicated a 320-bit bus, and 14gbps RAM modules. Thus the 560 GB/s bandwidth predicted. Note that the E3 layout strongly suggested 16GB total capacity. However, due to the layout it'd be relatively easy to upgrade RAM capacity as far as 20 GB if Microsoft wanted, which is why my prediction is "16+".