From my understanding, neither.

It's the same code (presumably) but Chrome OS's version was compiled with metadata that said it was running on Chrome OS and Linux's version was compiled with metadata that said it was running on Linux. And that metadata is later embedded into the license request which Xfinity's servers check.

Of course, that's just my assumption from what I understand. I can't verify anything due to how obfuscated Widevine is. But due to how long-lived this problem has been, the two Widevine versions having the exact same version number (4.10.2557.0), and that everything else Widevine-wise works, I'm inclined to believe that this is intentional.