That is because GPS satellites, orbiting about the center of gravity of the Earth, can measure heights only relative to a geocentric reference ellipsoid.
Awen fal as satelit tn win GPS tan, aqqalnen šund ebănɣ n ammas wan aɣălilwăy n amadal, adobăt eket n iddikud n izuk net daɣ as orak sa s ammăs n ellipsoid