The richer mix if for safety. the factory tune is built to run in many climates ... from desert to mountains. When you tune the car, you set it up for a specific climate and specific mods. It's not effective to do this at the factory.

Also, if Mitsu tuned the car for full performance from the factory you couldn't mod it at all w/o a tune. With the extra padding in the conservative factory tune you can swap out your basic bolt-on parts without risking blowing something up. when it's tuned you take that buffer out and small changes to the car can cause issues, which is why yo re-tune if you add/change parts.