when I'm downloading from Steam, I tend to get around 12-14mbps at peak, dropping to 10 at times. I suspect this is more to do with Valve's servers and/or setup than anything else, and that I tend to download at peak times.
Speedtest shows the full 100mbps I pay for
you have to consider the limiting factors at play, though. The availability and size of the file/s you are downloading, the capacity of the webserver, how many other people are using it, how many other people in your immediate vicinity (from your network or neighboring ones) are sapping up the area's bandwidth and so on.
Do you let Steam automatically choose which server it is downloading from or do you choose?