It is because earth is no a perfect sphere it is geodic. the polar ends are a bit flat and nearer to the core hence g is stronger when the distance betwee them is less
The earth is not a perfect sphere. The earth is more bulging at the equator than at the poles. Thus for any body, the distance between the body and the centre of earth is more at the poles as compared to that at equator.

We know that value of g is inversely proportional to square of distance between centres of bodies. As the distance between the centres of body and earth is more at equator than at poles, value of g is greater at poles than at the equator.
Hope you understand it
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