On my home network, I have the following: 

Speedtest.net: 250+ down / 800 up

Home Asus router: 800 down / 800 up

Home Google mesh run off the router:  800+mbps 800 up. 

Logging into century link (I use centurylink fiber) shows: 940 down and 800 up.  

We have all the streaming we care about with movies and so on, so again, I don't have any issues.  It's just a curiosity.

What could explain that difference between online speedtest and local versions of the same thing?