It’s the weakest link principle. Your speed depends on all the pieces that are part of the delivery chain.
That includes the internet connection between your computer, your router, your provider, their provider, their routers for different countries, the cables between countries, then all the providers backwards to the company that serves the content and finally their server(s).
The speed test is designed to only measure your line and is usually done to the most local server that does nothing than to send dummy data. Therefore there can be a huge amount of extra factors that come into play when you do a “real” test and it will differ from your “pure” result.
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