Again i just posted something similar a minute ago, if your 1.5km from the exchange it doesnt mean that your line goes straight to it, irf your getting a higher speed than calculated then good
However sky use BT systems to get this information and it may not be accurate. If your walls are slightly denser in another area of the house this will effect your wireless signal and ig you have alot of pipes in another wall again this will effect it.

If you have doubts about your line speed check it on SamKnows