As per the earlier post, its giving a prediction based on the BT Wholesale equipment, so will not necessarily give the same speed as an LLU suplier.
For example, you may have ADSL Max on your line from BT Wholesale and ADSL 2+ from your LLU supplier and be relatively close to the exchange. In that case, the BT Wholesale service might be capable of delivering say 5Mbps and the LLU provider 16Mbps.
It wouldn't make sense for the Wholesale line checker to give any estimate of performance using anything but its own kit as there are too many variables. Either way, it shouldn't matter as long as ISPs give an estimate at the point of purchase to allow an informed decision if speed is a key critieria to the purchaser.
That said, worth asking Sky why their estimate is so far out given they are supplying you, so ought to be able to check the actual speed of the line. I can't see this being a conspiracy though as Sky's estimate is wrong and they are not currently selling FTTC, so hardly in their interest to do so!
All opinions expressed by me are my own
(excluding 2Mbps for BT Vision)