Server Memory Speed Mystery

I’m hoping someone will be able to solve this mystery.

Despite setting the memory speed to 2133 in the bios, the output of
‘dmidecode --type memory’ in the Proxmox shell shows the memory is running at 1866.

The server is a Lenovo Thinkstation P500, Intel Xeon E5-2699 v3, 64GB DDR4 PC4-17000 (8GB x 8). The part number of the modules is 18ASF1G72PZ-2G1A2 which is:
• Type: DDR4 SDRAM
• Capacity: 8GB
• Data Transfer Rate: 2133 MHz (PC4-17000)
• Form Factor: 288-Pin DIMM
• Voltage: 1.2V
• Error Correction: ECC (Error-Correcting Code)
• Buffered/Registered: Registered (RDIMM)
• CAS Latency: CL15
• Organization: 1Rx4 (single rank, 4-bit wide data paths)

Is there a speed limitation because all of the memory slots on the board are full? Something else?

Does your CPU support a memory speed of 2133 MHz?

Yes it does according to the specs on the Intel site.

Intel Specs

Which motherboard is inside? Is your bios up to date?