Every time the ball hits a step it "squashes," or flattens a little, as a result of impacting a hard surface. As it bounces again it stretches or becomes somewhat streamlined as it sails through the air. What would be the point of animating a ball that stays exactly the same shape regardless of whether it's hitting the floor or going airborne? There would be no character - and no interest - to such a ball.
Similarly, characters faces squash and stretch in reaction to different emotional situations. Here I've drawn several of Blair's examples.
First this dumb mutt has a completely vacant look on his face. Then he notices something: "Huh?" Something that amuses him. Makes him smug and silly. And then something about it seriously pisses him off. So much so that he barks ferociously.
Heh heh. Dumb ol' mutt.