To use a very unscientific term, the earth is squishable. Put a heavy weight on it and the crust will deform. Remove the weight and the crust will rebound.

During the last ice age, the Laurentide ice sheet was extremely heavy. About two miles thick and extending over what is now Canada and part of the United States, it caused the crust to deform, essentially squishing it out to the sides.

That left less mass underneath it, and less mass means less gravity. Since the sheet retreated, the crust has been rebounding, which means gravity has been slowly increasing.

At least that’s what scientists have assumed to explain anomalous gravitational readings in part of Canada. But there could be other explanations as well, particularly tectonic processes driven by mantle convection, the flow of heat from within the earth.