speedtest sites

I have been consistently checking my speed tests through this thinkbroadband site, which since installation of a TT router instead of my 3Com router, have been slow. Down from 4.5-5mbps to 2.5-3mbps. Up speed seemed quite consistent at 360-370 Kbps. But I followed a link to another test site http://www.speedtest.bbmax.co.uk/, and then another http://www.speedtest.net/. With both of these the speeds read at 4.9-5.2mbps and up speed was similarly consistent. Is there a good reason why thinkbroadband site should differ, and which readings should I take as accurate for the speed my broadband is runnin g at.
Thanks v.much for any input and keep up the good work,
Barlingman
PS I purchased a new router from TT because although satisfied with the 4.5-5Mbps speed of connection it seemed to regularly lose the connection. I found if I restarted the router it came back again, usually. TT CS advised the new router for