Can someone explain this?
I would think it would be a better mixture because the exhaust flow velocity is increased.

 

Aftermarket headers make you run lean because by improving scavenging, they allow more air to be ingested by the motor. But since the speed-density metering system on Hondas is not able to compensate for this too well, not much (if any) additional fuel is injected to compensate. This causes a lean condition.