Actually, the OS version isn't reported that way, I don't recall how exactly but it's certainly not a string with the full commercial name.
This example of check here might help https://support.microsoft.com/en-us/kb/307394
As for the real reason, I'm not sure myself, of all the suggested explanations the one I found more plausible is that MS just wanted to get as far from 8 as it could, silly as it may sound.