As a pedantic note, I wonder if the cap is measured from your cable modem or access to and from the Internet. Because if it's the latter (ever how unlikely) then a P2P protocol could make a difference.
I suppose that scenario could exist somewhere. It definitely doesn't on Comcast in the US.
I mean this in all honesty: how do you know that?
Try a traceroute to your neighbour who is likely on the same physical Comcast segment as you. It's not a simple 2-hop route. It goes way out and comes back.