I would assume that the server the Hub connects to is within BT's network, and it's likely optimised to show as good a speed as you will get. Testing to the outside of their network will always be more prone to congestion and contention. I've seen enough results on other people's hubs that I'm not suspicious about it being a fair representation.
*I don't use the BT Smart Hub 2 that I have, instead, I use my own Asus GT-AX6000 and a wired device will quite happily connect to Ookla speed test sites at 940/114. The fastest wireless device over a Speed Test here manages a little lower in speed but most of the "power" clients here are either AX1200 or AX2400 (on the 5GHz band alone). Testing on a 5GHz AC866 PHY connection would obviously be slowed further as you need to account for both the slower speed and overhead on the connection.
I only learn by making mistakes and owning up to them - boy do I learn a lot!
