Interesting. Are ISPs known to throttle steam or something. I’ve noticed that steam almost never downloads at the same speed of get doing a speed test. I’ve noticed it many times through out my life, though admittedly I’ve been stuck either way the same ISP across many states.

Steam also extracts while it's downloading and will slow download speeds if there is a healthy buffer of things to extract. If you're downloading a game with thousands of small files to an HDD or even just a cheap SSD chances are you'll be throttled by your own computer's throughput rather than your Internet connection.

Steam downloads are often heavily cpu bound at higher speeds. I usually max out my 7800x3d at <1.5Gig.

Some ISP throttle sustained download, which explains having high test speeds and initially fast downloads that slow down with time. So it might not be Steam per se that they’re throttling, though it’s not impossible.