And the single D3.1 channel is downstream, there's never upstream 3.1 channels with VM's setup.
As you've found, the 32 downstream channels can deliver around 700 Mbps in practice, by bonding the channels together, you might think of that like a bunch of drinking straws. The single 3.1 channel uses a much wider channel width to add about another 600 Mbps of capacity, like a single big fat straw. VM's network operates a Gig 1 connection as the 32 ordinary straws sellotaped together for the downstream, four thinner straws for the upstream channels, and one single big fat one. If that sounds convoluted, it's all down to backward compatibility needs, and the fact that cable networks were originally designed to distribute analogue TV content, which was entirely a downstream traffic flow.
The field implementation of DOCSIS 3.1 is a miracle of ingenuity, mathematics, physics and engineering. It's also a miracle that this unbelievably complex Heath Robinson lash up works at all, but you can say the same of many technologies that only fully flower in their twilight years.