Speed test sites often pick a server near you to give maximum speeds possible.
Go on a web site and it may be far away and outside the carrier network and the speed plummets.
So while I think that's the easy explainer you will have folk ask again Why?
Why do speed test for mobile data indicate high speed(5mbps) but when browsing the speed goes low to below 100kbps. What is the difference between doing a speed test and web browsing