Earth is not perfectly a sphere, but somewhat flat at the poles.

The value of acceleration due to gravity, varies from place to place at the surface of the earth and also as we go above or below the surface of earth. It is due to two reasons, shape of earth and axial rotation of the earth. Earth is not a perfect sphere, but somewhat flat at the two poles. Its equatorial radius is nearly 21 km larger than its polar radius. Also,

Here, G is gravitational constant,
M e is mass of earth and
R e is radius.
Clearly smaller the value of R e large is the value of g.
As we go from the equator towards the poles,
the radius R e goes on decreasing and hence the value of g goes on increasing.