[gwillen]

Their docs say (somewhat recently updated, I think) that even 0 is not perfectly deterministic in all cases (though it's very close). Some people had previously observed this and speculated that it was some kind of floating point roundoff issue, when two outputs have almost identical scores.

[Jensson]

GPU's often do floating point computations in nondeterministic order for performance reasons, so you would get small differences even for exactly the same order. It is probably that.