Perdu - that's because it's a very simple test, which downloads a file (500k or 1mb) to your system, and times how long that takes. Your browser is smart enough to know the second time that you have already downloaded the same file, and has kept it on your computer, (in your cache), so doesn't bother downloading it again.
To re-run the test you need to delete temporay files (somewher eunder Tools/Options). You probably won't want to delete your cookies though. (This will free up some disk space too)
