Here's the display:
These are relatively new servers, so there was nothing especially helpful on the web. Other people had this problem, but I couldn't find any real solutions, other than a reference to C states on the CPU.
I experimented a bit and figured that CPU power savings might be the problem (I'd seen this before on Opteron servers.)
The fix is to set CPU performance to HPC (it defaults to Enterprise) in BIOS:
That's advanced -> CPU Performance -> HPC (from Enterprise)
This solved the problem.