Actually, a VPN isn't completely unrelated to a speed test.
A lot of ISPs do "traffic shaping", which is prioritizing certain kinds of traffic over others. A few years ago there was a lot of speculation that Comcast was cheating on speedtests by detecting traffic that looked like bandwidth testing, and prioritizing it. This makes your connection appear faster than it would be normally. If you've ever had slow Internet but a fast Internet speed test, this could be the reason.
A VPN can help hide your traffic type from low-effort traffic shaping, and thus help prevent that kind of cheating.
I agree. Couldn't a VPN service do the same thing? Can you draw conclusions from different results?