Can you run several tests: Chrome, Safari, Chrome, Safari, Chrome, Safari, Chrome to see if there is a consistent difference?
I have noticed that running SpeedTest on a 1.3 Ghz dual-core laptop gives lower speeds than on a 3.0 Ghz dual-core laptop -- both using Ethernet cables, not WiFi. To me, this implies that the speed of the computer, AND the speed of the web-browser, affects the results of the SpeedTest.
So, how fast is your computer? Does the "Performance" tab on the Windows "Task Manager" show your CPU running at 50% or 100% while the SpeedTest is running?