Internet speeds/speed tests accurate?

The answers if speed tests are reliable is no. No they are not. There are many factors that go into speed testing. One is traffic one the server that is being pinged. Another is distance to the server. Another is the route that is taken to get to the server.
When testing from your xbox one you dont get to choose the server. I can run three different speed tests to three different servers and end up with three very different results. Some servers dont have as much bandwidth either which can cause poor speed testing.
That's why most companies do not guarantee speeds. The way I gauge mine, I use the same server almost all the time. here is an example 3 different servers very different results. Upload remains the same because I run much faster speeds but my modem is maxed
out. (Need to upgrade)

You can check the Microsoft Xbox Servers, they are currently up and running. Have you tried changing your DNS settings for a more accurate connection in your region? If not then do so. It will help a lot.

I use my Xbox One X to make the test (cable) and the test are right.

I have 500 MB/50 MB and the console shows me 490 MB / 49 MB.

Are you using cable to test your speed? What type of nat you have?

Regards!

I think, in some cases, it may actually depend on your internet provider/service. I've thought about this before back when we switched ISP's in my area and everything was pretty much accurate verbatim. It wasn't until we actually switched providers that
my readings started to appear wonky and disproportionate.

Yes I changed the DNS settings and it was reading the same speeds. However, I did not try online gameplay to see if there was a difference :/