Some ISPs have been known to recognise the speed check data type, or the Internet address, and to slightly prioritise the connection while it is taking place, thus giving an optimistic result, which looks good for them. When I first connected, over the phone line, I had a cheap 'starter' rate which gave a download of 2 Mbits. When this finished, I went on the normal rate, which was around 10 Mbits variable between 8 and 20 with no changes at my end, so the ISP is quite capable of adjusting the rate for individual customers.
If you use the speed check from Netflix, the data stream is biassed to be like video streaming, to give a realistic figure for their usage. It is what they do after all.
With the roll out of fibre to the home over here, it is quite noticeable in house sale adverts that the availability of fibre has become high on the list of features. Apparently it is also forming part of the sale contracts, with the option to withdraw if it turns out to be false.
Who needs Gigabit anyway ?
Andrew.