The ranking system was based on base 60 which was the basis of the Sumerian number system. The top god Anu or sky father was ranked 60 and his spouse Antu was ranked at 55--that pattern of husband-wife continues down the list. The other gods and goddesses were based off of that.
I'm surprised that greater goddess Inanna was ranked lower. The Greeks split Inanna into three goddesses: Athena, Aphrodite, and Artemis. Other civilizations called her Ishtar and Isis. After the Sumerians, other successor civilizations like the Akkadians, Babylonians and Assyrians elevated or added their gods to the list. Inanna went down in the power rankings. Inanna should be just below Enki and his wife Ninki which should give her a power ranking of 30 instead of 15.
Finally, the vast majority of Sumerian gods and goddesses were given a power ranking of 5. For instance, The mother of Gilgamesh, Ninsun "Lady of Wild Cows" and the patron goddess of Lagash, would have a power ranking of 5.