[_bxg1]

I could imagine algorithmic differences adding bias in error margins. Both versions might be accurate approximations of the answer, but one might lean towards one end of the error space and one might lean towards the other.

It's like when fixing a bug in library code breaks application code. Usually it's because there was some undefined behavior in the library - which wasn't part of the contract - which the application (knowingly or unknowingly) relied upon, and then the updated version produces a different undefined behavior.