I have a hunch it might be partially due to vendors being willing to fudge their numbers for the sake of marketing. Selling into the gaming market, label it 36

I have a hunch it might be partially due to vendors being willing to fudge their numbers for the sake of marketing. Selling into the gaming market, label it 3600 and if it crashes, they'll just tweak the clock and keep going. Selling into the server market, customers have a different expectation. Also many DDR-3600 modules can only run that way if they're the only DIMM on the bus. Populate two and you're down to 3333 or slower. Nobody runs a server with one DIMM, so again, the faster number simply doesn't apply to the ECC market, even if it's the very same chips. (Get more than two on the same channel, and you need to add buffers to drive the trace capacitance, which adds latency.) This suggests, perhaps, that you might be able to get some ECC DIMMs and overclock them all to hell and gone, if you're only running a single one per channel.