Usually ISPs have bandwidth throttling software because they oversubscribe. This means that an ISP has for example 100 mbps on their backbone, and they sell 20mbps connections to 50 people.
The ISP limits each customer's connection to 20mbps as a max and they know from experience that not everybody will use the full 20mbps all the time, so, they can get away with it.
The average oversubscription for ISPs used to be 7 to 1, I'm not sure how it is these days, but they all still do it for exactly that reason. They won't pay for bandwidth that is never used, it's bloody expensive!
So to control everything in the peak times, they have software running to make sure that not 5 users have their full 20mbps and the other 45 users have nothing. That same software can control you connection so that when you go to a speedtest.net (or similar), you will get temporarily your full bandwidth, and that way they can always claim it's the 3rd party that doesn't allow you to download any faster.
Please do note that, sometimes this is the case! Steam does throttle bandwidth, but they do allow more than 700kbps.
Also, you should check your Steam settings. There is a setting where you say what connection you have, set it to what best matches the reality.