It's because (until the new BT infinity stuff comes in) consumer broadband is asynchronous. You have a single channel for data travel along which is shared for both up and download.
It's essentially like a narrow bridge with traffic lights controlling the flow of traffic on it. As most people want information to come to them ASAP, the traffic lights are set to allow more traffic along one side than the other. So in your case it's 10 cars per second across from download then it allows one car per second from the upload side.
It's really that you have an 11Mbps connection shared with 10Mbps down and 1Mbps up
Though that's still actually pathetic. We have a 10Mb connection with Virgin and I can manage 1.5Mbps up and my connection is bloody slow. I'm sure it's supposed to be 2Mbps, but maybe even 4Mbps.