It ought not to click, because it's a deep objection. In the ball example, the wavefunction is a sum of two components: there are two possible measurement outcomes. How do you get three worlds from two states?

Say you have an electron which is either up or down, a 2/3 percent chance to be up. The wavefunction looks like `2/3 * up + 1/3 * down`. In the MWI picture, when you measure the electron, you split into two worlds: how do you get 2/3 probability of "up" from two worlds?

You can say you split into three worlds, two of them identical. But that is not found in the Schrödinger equation (so now you have new postulates, which MWI hoped to avoid), and what if the probability were 1/pi, how many worlds would that be?