[shadofx]

Cyanide blocks both the mitochondrial ability to take in pyruvate and its ability to generate power, which means the cell can still attempt to use the excess pyruvate to generate energy by fermentation, resulting in lactic acid which stimulates pain receptors.

Carbon monoxide poisoning on the other hand only interferes with oxygen delivery, which shuts down the mitochondrial ability to generate power but critically does NOT stop it from taking in pyruvate. Since there would be no excess pyruvate in the cell outside the mitochondria to undergo fermentation, there would be less lactic acid, and less pain.

That's just my guess.