Sometimes elevation is measured against the center of the earth. The problem there is that the earth does not really look like a ball, it is slightly squashed (an oblate spheroid). This means, that while still at mean sea level (at the equator), some points are farther away from the center of the earth than others. That way the peak of Chimborazo, a volcano in Ecuador is farther away from the center of the earth than the peak of Mount Everest. Yet, Mount Everest is higher above sea level.