//The actual speed depends on several factors such as the product option you choose, the processing power of your computer, how many people are using your broadband connection at the same time, whether you use a wireless or cable connection, the speed of the websites that you visit and the performance of our network. Speeds to be lower at peak times – peak times are evenings and weekend.
In the first ten days you may see your speed go up and down. When using a postcode only (rather than a telephone number) the checker results are less accurate. We use a best guess of where you live based on the centre point of the postcode and not the exact routing of the line to your address, and so the result must be treated with caution.//
From BT's Website Ratter.
http://www.productsandservices.bt.com/products/speed-checker/?s_cid=con_FURL_speedtest&utm_source=ATL&utm_medium=FURL&utm_content=A&utm_campaign=speedtest