Different ways to measure speed

I have a fibre connection, nominally rated at 300 Mbps download with Zen.

When I run the Think broadband speed test I get a download speed very similar to the nominal.

When I run the BT wholesale speed test I get about 1,200 Mbps download.

Can someone explain why ?

Given that your connection to the ONT is a gigabit ethernet, which only supports about 940Mbps of IP traffic excluding headers, then the BT wholesale test is clearly just giving nonsense results.

Why? Presumably it's just buggy. But if it's a browser-based test, you could try it in another browser just to see if that makes a difference. You'd have thought that BT would have tested it *somewhere*.

Browser based tests can be affected by caching, try a Private / Incognito / InPrivate window. Also if you are on Windows , some security products can affect the speed test results.