We know that the earth is not spherical in shape; the earth is flat from the pole side and bulging from the equator making the shape of geoid.
Thus earth is geoid in shape.
When we talk about poles, it can be clearly depicted from the diagram that the radius at the poles will be less than that of equator.
We know that,
It depicts that acceleration due to gravity (g) is inversely proportional to the radius.
Thus at equator the radius is more so the value of g will be less and at poles the radius is less, so the value of g will be more.