The value of
The Earth is a geoid with an irregular shape (think about mountains or oceans) and the field that generates around follows its irregularities.
For example the acceleration on the poles is
Of course the variations are not so big because you need a big mass to give some contribution to the gravity.
So the acceleration on Earth is not a constant, it changes from position to position and it changes in time because the Earth is continuously reshaping (that is redistributing the mass) under the influence of earthquakes, vulcans, wind, rain, etc.
The variations are small because the total mass of Earth is quite stable.