I suspect that your line has a configured limit as opposed to physical limit of 250mb/s (perhaps the fibre equipment is capable of 500mb/s or something).

I've seen this ever since I moved to fibre about 8 years ago, I've never taken the fastest packages and whenever I test on speedtest.net it always starts off exceeding my limit, then comes down to my limit over a short while.

The limiter for you may very well allow you to burst for a limited duration, so your speed on 1MB files are possibly at 440mb/s, but once you have sustained line usage it brings you down to your limit. I wouldn't know if this is an intentional feature or just because of how the limiter solution is implemented that it needs to sample over a certain period of time.

It is a slightly useful behaviour if you're browsing the web and only occasionally clicking links which land up being downloaded way faster than your normal limit, but as you're only getting this speed if you do little to nothing for the 10s or 100s of seconds between opening pages, it doesn't have any negative effects over all.