There is a very good beginners' read at
http://www.gcscte.org/presentations/...CSIS%203.1.pdf
I think in the described number of channels in this thread, insufficient have been added for DOCSIS 3.1 (a further 32 at least I would have thought). So I suspect these are just for the DOCSIS 3.0 DS pool ahead of the 1Gbps roll-out.
The DOCSIS 3.1 downstream frequency extends beyond 1.2GHz so there would be spectrum on the totality of DS channels for some DOCSIS 3.1.
The above is not authoritative - just my take.