Originally Posted by Eivind E

I know assumptions can easily cause misunderstandings, but usually people with this kind of extreme mileage issue, will have a very fat bottom end tune, which causes them to be on the throttle much more than they should. This lowers runtime significantly.

Check that all your exhaust seals are good, then make a new attempt at tuning it, with the goal of having a crisp, clean bottom end. An excessively lean top end can mask a fat bottom end. So before you tune the bottom end, open up the top 1/4 turn (or more if needed!).