Advertised sownload rate or speed is much like fuel economy. yes you can get 58mpg with a 9 st driver maintaining exactly 56mph on a dry road with no wind and nothing in the car at all except about 2 gallons of gas and maybe even with a few non essential bits taken off.
I live very close to my local BT exchange-cable route-fibre optic to the curb is 300m and I get 76mbs . BT advertise 80 but you would only get that in the exchange itself .
When trying to find out what really works you need to find a fairly stable site that you can download something from - say a video clip or tune where it tells you the file size. If you know where the server is you can run a test download at different times of the day which will take out the contention issues if its an unsocial hour with you and the server. 0600 uk is 0100 EST in USA so you are likely to get a better result than at say 1900 uk 1400 USA.
And never try any test when its close to midnight in California or US Central time since these are the times when all sorts of little fixes and mods and changes get done on the network so consistency isn't going to happen.
If you run your download time at the off peak times suggested then you can in parallel run a BT speed test or Ooklas one and see what speeds they show at that time of day and you can compare that with the actual download to give you some idea of the gap between advertised mpg and what happens in your car day to day.
If you do get consistently low speeds go back to your ISP and complain and explain you have tested it in the wee hours of the night and compared it with downloads done at quiet hours. There are all kinds of things that can go wrong with the local loop ( exchange to your house) bit especially if you do not use BT. BT still run all the cables so everything comes down to them and they only give the actual ISP outline info on loop distance and performance and this can easily be wrong due to changes in cabling or equipment in the BT part of it which do not get transferred to the ISP.
BY the way overhead cable or underground shouldn't really make any difference. But satellite is a basically a dead loss , data protocols do not like the delay inherent in satellite connections and seldom work well which is why satellite are not used for international communications anymore.
Your ISP can change the line speeds and some of the line characteristics from the defaults BT give them and that can be worth a try and the route a cable takes to your house can be miles-literally-different to what you see above ground. Some years ago I swapped to O2 and got slow speeds which they said was because the cable route to my house was 3000m whereas it is actually 300m, either e typo in the info BT gave to O2 or the fact that BT altered the cables at some point.
So its real problem if you don't live close (2500m or less ) unless you use BT Infinity which terminates at the nearest cabinet (green box on pavement) to you.
Sorry to ramble on a bit but I have found that a bit of research pays dividends on this issue and not taking on trust what either BT or your ISP say without challenge
Good luck