Title explains it all- this is a super weird phenomenon.
I use CoAX and have noticed that when I install a game from Steam I get variable download speeds (usually something like 27mbps). If I run a speed test I also sometimes get 27mbps. However, running speedtest.net actually increases my internet speed when testing- all of a sudden on speedtest I get my 800 or so mbps and steam starts downloading much quicker.
How and why can that be? Its really really weird.
Its also only on Moca and Coax I think, I think the wifi speeds are always 1gbps so I doubt they are scamming me