That's a good question! It can be surprising how objects that seem very similar, like two basketballs that both appear to be fully pumped up,can behave differently. How high a ball bounces depends on the elasticity of material it's made out of and, in the case of balls that need to be inflated, the elasticity can also depend on how fully inflated the ball is.

Elasticity is basically the ability of an object to bend without breaking and then to return to its original shape. If you were to drop a basketball and a lump of clay, it is the elasticity of the rubber basketball that allows it to bounce back while the clay just lies on the floor. Both objects can bend without breaking when they hit the ground, but the clay can't return to its original shape so it can't bounce back.

The bounciness of a ball is given by a number called the "coefficient of restitution", which is just a big scientific term for how high a ball bounces after you drop it. A super ball, usually made of highly compressed synthetic rubber, has a coefficient of 0.9 to 0.95, which means that when you drop the ball it bounces back to 90% - 95% of the original height that you dropped it from (almost all the way back!). A fully inflated basketball, on the other hand, has a coefficient of about 0.75, which means it bounces back to 75% of the height that it started from (3/4 of the way back). When a basketball has a little less air in it (even if it's such a small difference that you can't really see it),the coefficient will go down and the ball won't bounce as high. This could be the reason that some basketballs bounce higher than others even when they both seem to be equally pumped up.