That measurement is for entertainment only. The Internet speed test finds the closest server and checks your upload and download speeds. In the real world, your data comes from several "hops" (links) and the speed you get is determined by the slowest link. The speed test will give you the speed of the last link and that will usually be higher.
The slowest link may not necessarily be permanent either as sometimes Internet traffic may slow down a link. The real speed of the Internet varies because your data does not always come from the same server and even if it comes from the same server, it may not take the same links. Even if data comes over the same links, the links may not always have the same traffic.
So what you have is a chain and all you know is how strong the last link of the chain is. The real strength of the chain may not be determined by the last link. Even the test selects your server among many competing servers and probably finds the one that does not have a lot of traffic.
