Using your example of riding across the mountain, as the actual altitude increased, you experienced less and less power so to maintain the same speed you were running at a lower elevation, you probably twisted the throttle a bit more to compensate for the loss of mph. The air was not as dense at the peak of the mountain as it was at the base. Since you were dealing with an internal combustion engine that required not just fuel but a proportionate amount of air for it to operate at peak efficiency, that imbalance due to the less dense air slowed the bike down. Of course other factors were at play including barometric pressure and temperature which also impacts the oxygen content of the incoming air. That engine is struggling to make power until the tuner makes corrections for the deficiency of elevation but you can retune to bring some of that power back from the losses of high altitude, but not all. DA calculations bring into account all current weather conditions so a tuner/ racer can know exactly how it alters the "real elevation" because that internal combustion engine must now operate based whats going in through the induction system. If the DA tells me my real elevation of 190 feet above sea level is now 2500 feet, I know my bike will not run quite as fast due to a lack of oxygen and I now need to either re-jet, or change my dial in because my once efficient "pump", will struggle to produce the same output.
