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?

Intel C612 and yes, up to date. This is the mystery, 2133 should be supported, and that is what is configured but it’s not using it.