From the Moon, the Earth looks quite round, though is it anything but round. The Earth is lumpy, all her land mass on one side so that her gravity center is a bit more landside than seaside. And with her waters bulging at the equator, she tends to be squat with a bit of spread about the middle, so a circling satellite would appear to be lower over the equator than in fact it actually is, in relationship to the gravity center.
Satellites intended to go round and round over the poles, or anywhere but the exact equator, must be moved out of the circular path around the Earth's gravity center which they naturally assume. The satellite when launched indeed hugs the gravity center of the Earth uniformly, circling the lump, but this leaves the satellite masters unhappy as the satellite is not a uniform distance above the Earth's surface, be it land or water. They then puff the satellite about until it is not circular about the lump, the gravity center, but is a uniform distance from the surface, be it land or water, in other words, is an ellipse. The reason they have so much trouble achieving a uniform distance from the surface is not because the satellite wants to go in an ellipse, but rather the opposite, because it wants to go in a circle about the gravity center of the lumpy Earth. It has to be tweaked out of this into an artificial orbit, into an ellipse!
Earth's center of gravity may have been nicely centered when she first congealed after the local Big Bang, but she is as centered as a three legged elephant now. The collision that sent her out of her former orbit in the Asteroid Belt and into the orbit she is currently in left her as oddly shaped as Halley's comet, and only her waters, pooling evenly about her, covered her scars. Over time she has molded herself, during repeated passages of her big brother, the 12th Planet, so that her molten core is circular, but her crust is anything but even. Depth of the oceans and distribution of the land masses is not all there is to it, as the composition of the crust must be factored in also. Lava flows from the depths have a different composition than the original crust of the Earth, heavier, and where these flows have spewed forth repeatedly during traumatic times, there is a gravity weight in that direction - the sum of the parts, wherever they may lie