Result given by mybroadbandspeed.co.uk is not accurate

[#1]

The speed tester mybroadbandspeed.co.uk does not give accurate speed results.
My router sync speed is 960 Kbps, my BRAS profile is 0.5 Mbps, mybroadbandspeed gives results that vary between 615 Kbps and 1199 Kbps (which is impossible).
The speed tester on thinkbroadband gives more reasonable figures (varying between 0.4 Mbps and 0.5 Mbps).
It looks like mybroadbandspeed measures the peak speed within an interval rather than averaging over that interval - so if packets get clumped together due to queueing the speed is significantly over-estimated.

[#2]

Welcome to the forum gc1
You're not running Kaspersky are you by any chance?

[#3]

No, I run avast!
But I am on a long telephone line, mybroadbandspeed.co.uk reporting 139 - 179 Kbps down speed and 355 - 359 Kbps up speed today with a router sync speed of 928 Kbps. The thinkbroadband speed checker reporting 117 down and 375 up.