How is change possible only in one of two deterministic, parallel unviverses?

Complete noob here.

I’m reading a book called "The Beginning of Infinity" by David Deutsch, and he describes an experiment in which two identical universes in multiverse suddenly become different.

He states that they are deterministic, and the moment when they start to differ happens after some voltage pike in one of the teleportation booths.

How can this happen? Doesn’t the fact that they are deterministic doesn’t mean that the voltage should be exactly same in both of them?