With a speedtest.net result you're only sending tiny packets of data from one system to another. When you download a game from Steam etc. you're downloading a whole game (lots of bandwidth needed) from the source. The download speed can depend on the source and many other factors, not just your own download speed. Also the internet provider is mentioning for instance that you'll get certain MB speed, in reality you won't. That's why they mention 'upto xMB' so this way they can get away with providing much less. Try changing your service provider or upgrade t a different package.