"17 something or others" refers to the maximum speed you could wring out of the system. The way they word it is... "Up to 17 something or others". For you to actually achieve that speed you would have to live inside the telephone exchange. In reality the further from the Exchange you live, the less you get. If you go for Fibre optic broadband, the drop off in performance is less noticeable as you only see degradation from the cabinet. So, the further from the Cabinet you live, the less you get.
Fetere sum, ego sum Ergo