I posted a similar question which can be found at this thread...
http://bbs.adslguide.org.uk/showflat.php?Cat=&Board=virginnet&Number=2931005&page=0&view=collapsed&sb=5&o=2&fpart=
Save them all posting answers on this thread for you.
The simple answer that was revealed is that the BT test site gives you true readings as it is uncongested while it's connected to you, but most of the other sites are congested and hence slower in responding to your pings.
Mike.
Edited by deleted (Sat 10-Mar-07 08:32:00)