First and foremost: Consider yourself lucky.
We are talking about MegaBit, right? Because, sometimes, speed is advertised as MegaBit/Second, while downloadspeeds are often shown as megaByte/Second by the Operating System (which is not the same).
Some reasons can apply:
- You are connected via cable, and your provider does indeed guarantuee 100 mBit, but allows more, if capacity exists. This is VERY unlikely, but may happen once in a while.
- You see caching effects
- Your operating system does not count in a correct way.