Depending on what else is going on with your network, that could affect your speeds. If other computers are connected, if you are browsing the internet, playing a game, etc. This can all affect the speeds.
Also, depending on what you are downloading from it could be an issue. Downloading torrents will vary. Downloading from a dedicated server should provide the best results.
You can use toast.net and use the ISP speed test to see what your throughput is to compare to other ISPs as well.