"Correct figure" is a difficult term.
Every actual speed test from any source is only a snapshot at the time of the interaction of many variables over which the test provider has no control. The provider minimises these as much as they can, by (at a guess), two main methods. Installing more throughput and processing capacity than they can reasonably expect, and setting up peering links to the main centres and ISPs in their own country.
If you have ever used speedtest.net for instance, the server it chooses automatically for you by testing a few latencies to you I find almost always gives me a lousy actual speed. It also rarely chooses the same server on two different days.
I use one I choose manually that I find consistently gives me high speeds, as it has to be nearer to "correct" that the lower ones. I also use the thinkbroadband one, and am pleased to report that when the two are used within minutes of each other they are generally in close agreement.
Even when the server company has the best possible setup as described above, the test data still has to go through your ISP and many intermediate routers all with immense amounts of other traffic to handle. For instance, extreme cases are when a Wimbledon final is on or a popular figure is playing, and a Formula 1 race is on, or the common clash of the Boat Race some other sport occurs, a speed test is likely to be poor.
Though the major ISPs are these days installing orders of magnitude more throughput capacity as the cost of it per gigabyte has dropped dramatically. So the effect of such clashes is decreasing.
My broadband basic info/help site - www.robertos.me.uk.
Domains, sites and mail hosting - Tsohost
Connections: OnePlus 8 Pro max 165Mbps down, 24Mbps up on Three, and B311 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
To argue with mindless bigots is foolish.