I use the term unpredictable behavior simply to describe a case where the contract has behavior that was not fully anticipated by at leat one user of the contract.

Of course, the code is deterministic and so the behavior was unambiguously defined in the contract's source code.

But there exist all sorts of semantic bugs, which can at times be pretty subtle, which means that even for an expert, the mental model of code execution may not match reality.

If there were nondeterministic behavior in the Ethereum VM that would necessitate a fix to the VM and would likely be completely uncontroversial.