Puzzled about Speed Test Results

Hi Folks,
I wonder if any of the "wise ones" can clear this up please?

During the course of over three months, the think broadband speedtest has given me fairly consistent results (15 tests) for download of between 2.00 and 2.7Mb/s. I've then immediately tried other speedtests such as the ones at speedtest.net or broadbandspeedchecker.co.uk> Both of these two tests report my d/l speeds as between 4.2 and 6.5Mb/s, over this same period!
Anyone any thoughts?

Thanks
Puzzled Mike

Disable your antivirus when testing @speedtest.net or broadbandspeedchecker.co.uk.