Gravity does vary depending on where you are on the earth due to the planet not be perfectly spherical. It is also affected by the centrifugal force which appears to reduce it at the Equator.
In combination, the equatorial bulge and the effects of centrifugal force mean that sea-level gravitational acceleration increases from about 9.780 m·s−2 at the equator to about 9.832 m·s−2 at the poles, so an object will weigh about 0.5% more at the poles than at the equator.
This page explains it more.
http://en.wikipedia.org/wiki/Gravity_of_Earth