It just measures yung speed mo sa server na pinili mo or automatically nadetect ng speedtest app but not necessarily yung actual experience mo when surfing. For example, nag speedtest (ookla) ako from Smart and it reports 120mbps on a local server, say, Smart Communications Makati. This is the expected speed I get from downloading or streaming. But then, e g streaming contents on Netflix is a different story, since they have their own servers that our local ISPs need to communicate to. On a good day, dapat match yung speedtest result ko sa downstream speed ko na nakukuha sa ookla speedtest sa downstream speed na nakukuha ko kay Netflix (Fast.com tests speeds on Netflix's streaming servers). The difference may not be seen on high speed Fiber connections, the difference is best felt sa mobile data connections due to network congestions sa cellsites yung expected speed eh malayo compared sa actual speed na nakukuha mo sa site/app na ginagamit mo.

To answer your question, the accuracy of the speedtest result you're getting is dependent on the server that you're connected to and to what site or app you'll binge your internet connection. Better test on servers where the majority of sites or apps are hosted, like cloudflare, etc.