That speedtest will show you the speed of which your router is connecting to the internet. The thinkbroadband speedtest shows the speed of which your computer/device is connecting to the internet.
Therefore your computer/device is not utilizing the full amount of bandwidth you have.
Are you using a wireless or wired connection? All speedtests should be done on a wired connection
After checking to see if you've got the latest driver(s) installed for your network card, and you find there are no updates available, could replacing the card for a more up to date one possibly make your computers speed more in line with the routers speed ?