Oh, urandom is almost always non-deterministic. It mixes completely
random bits from hardware sources with its pseudo-random number state.
The more random bits it gets from hardware, the less predictable its
output is. However, as long as it's getting any random bits, it's output
is not deterministic (because it's based on some random information).

But perhaps there is better wording that conveys the power of the
urandom source?