speed test education

I do not have a speed problem but I am just looking for reasons why speed tests give different readings.
I have 900Mbs to the router but using other speed tests can get anything fron 300 Mbs to 900Mbs depending on the tester. Sometimes if I change server on the test site the speed sometimes rises.

It depends on more than just your link. If your link can do 900,but somewhere along the route between the speedtest server and you, speeds are limited to 300, then 300 is all you will see.