this is a hella nerd response...

but you said the elevation is really high...so maybe the air itself has less oxygen...I know that at extreme elevations, oxygen partial pressure can drop from the usual 160mmHg at sea level to around 100mmHg...maybe that can play a part in your engine not burning as efficiently as it should because there just isn't as much oxygen to combust. I'd say, ask the people who live there...see if there's another honda around and see if they get lower than expected mileage too...

i would think that the ECU can sense [O2] and compensate...but iono...just an idea.