Well, of course at the real hardware level there will be some non-zero amount of nondeterminism, but the error correction steps that have been implemented are enough to make the probability of it behaving contrary to the design of the hardware, low enough to be pretty negligible.

And if you want additional assurance (due to, e.g. the computation being very long) you could probably use some cryptographic techniques to produce a certificate that the output is as it should be, and you can check that certificate as many times as you want to avoid possible random hardware errors in the process of checking the certificate.