1.5 times greater speed comes from 768-bit vs. 512-bit width of the memory interface.
Another 1.5 times greater speed comes from DDR5-4800 vs. DDR4-3200.
The rest may be from virtualization and other overheads.
Number of DIMM is important, I know the standard BOM at my company has way more memory that we use just to have more bandwidth.
Bandwidth increases nearly linearly with adding dimms until you get to 1 dimm per memory channel. After that there's very small improvements related to more open pages. More open pages doesn't help with pure bandwidth benchmarks though.