There are variations in the value of acceleration due to gravity on the earth because of variations in the earth's shape. Gravitational force depends inversely on the square of the distance between two objects. The distance from the surface of the earth to its center isn't constant. Features such as mountains, valleys and the equatorial bulge result in small variations in the earth's radius.
The earth's rotation also causes variations in its gravitational field. Centrifugal force is greater near the equator than at the poles so the effect of rotation isn't uniform,
The earth's not quite spherical shape and its rotation cause the gravitational force at sea level to vary by about 0.5%. Variations in altitude cause it to vary by about 0.29%.
The value 9.81 m/s^2 is the average acceleration due to gravity at sea level.