Before you start investigating, make sure that you're consistent in comparing your units. Many programs, such as download clients, will report speeds in terms of kiloBytes per second. Here on Speedtest,net, we default to the industry-standard megabits. For example: 100 kilobytes per second works out to 0.8 megabits per second. You can have Speedtest.net do the calculations for you by changing the units on your Settings page.

If the discrepancy persists, this can happen if anti-virus or firewall software on your computer is intercepting test traffic. Examples of this type of software include McAfee, Norton, and AVG. A properly configured firewall shouldn't cause issues with the test, but you can try momentarily disabling the software before a test to see if it's causing the problem. (Don't forget to enable it again afterwards.)