You are most likely just one of many hosts that are sharing that file.

What happens is the downloading host only gets that part of the file that is needed from you. The other parts are gotten from the other hosts sharing that that file. This is what is refered to as the swarming effect.

It is the same when you d/l from many host that have the same file. You get pieces of the file from each host, thus decreasing the the amount of time to d/l the file.