If computers were fully deterministic, we wouldn't need error correcting ram.

The abstracted design of the machine is meant to be deterministic, but you can't predict before running any command whether or not it will complete because there are externalities that effect the outcome.

Electromagnetic interference even happens in-chip where an electron can accidentally escape it's wire and enter another, possibly resulting in an error, but not every time.

It's even been used as an attack vector where rapidly flipping a bit increases the likelihood that a neighbor bit is also flipped, but the method is probabalistic, not deterministic.

Yes. In theory they are deterministic. In practice, not so much.