The earth is slightly flattened from a perfect spherical shape because of

isaac newton first proposed that earth was not perfectly round. instead, he suggested it was an oblate spheroid—a sphere that is squashed at its poles and swollen at the equator.

the idea of the earth being a flat disc is also inconsistent with the fact that a given lunar eclipse is only visible from half of the earth at a time. the only shape that casts a round shadow no matter which direction it is pointed is a sphere, and the ancient greeks deduced that this must mean the earth is spherical.

Its way of rotation.

As earth is rotating at an incredible rate, the equator swells outwards because of the centripetal power. Thusly, the posts will look all the more level in contrast with the equator. For instance, when the earth is turning, there are inward powers attempting to dismantle everything to within.  

Since, the powers are continually acting towards the focal point of the earth from all bearings, with least opposition from shafts and the greatest obstruction from the equator, the net internal pressure is more for the posts and less for the equator.