There are so many variables that go into the hook potential of a ball, including several that aren't related to the ball, that all these rating have to be taken with a grain of salt. Examples of extraneous factors are lane surface, oil volume, length of oil, along with rev rate and axis rotation of the bowler. The result of the inter action of all these can cause one ball to hook more on one condition, and a different ball to hook more on another condition.