See, Skippy added a "dimple" to the bottom of the jar, which means that instead of having a flat bottom like a plastic jar should, it's concave -- it goes inward, like Jennifer Garner's cheeks. And the weight of the jar, which was previously 18oz, became 16.3oz. The price did not change. I'll repeat: the price stayed the same, and you got 9.44% less peanut butter.
As for the other peanut butter brands, Jif struck back by keeping its jar the same, and even enlarging the font on the jar, to let choosy moms see that it was still 18oz, instead of the 16.3oz Skippy offered. That didn't last, as you'll soon find out.