Getting more than allotted bandwidth on speed tests

So I have a Siti 1Mbps plan. On speedtest.net, I get speeds of around 1 mbps all through the day.
But on other checkers, such as Bandwidthplace.com and http://www.speakeasy.net/speedtest/ I get D/L speeds of nearly 8 Mbps
Any idea why this is happening?
Thanks