The radius of earth in poles is less than the radius in equator, this is the reason of being max. gravity in equator and min. gravity in poles.
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The planet of earth is not perfectly spherical. It is slightly spherical on the average of  North and South pole. Gravity is stronger when u r closer to the center of the earth, so if the poles are flatter than the rest of the earth's surface they would be closer to the center of the earth therefore making gravity stronger at other points.
Hope this helps u !!!!!!!!!!!!
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