(((interference)))
Veteran
Although Richard suggests that there may be an additional 1GB for game DVR on top of the OS reservation, so I don't think there is enough info to definitely say what it is. Sounds like it has been locked down though, and hopefully that will leak for more certainty.
That's actually Shifty's theory lol, I was telling him about it and I'm guessing that's where he got it from.
He seems to have got confused with the reason for having the buffer though, it was to save on HDD wear and tear (since the system would be writing to the HDD constantly when playing games) rather than saving bandwidth (where it would be a pretty insignificant saving)