With a speed test you measure the bandwidth of the link with the lowest capacity between the test server and your device.
One of the characteristics of an excellent speed test is that the speed test is accurate and consistent.
However measuring internet speed is hard. Hard for the ISP and hard for the customer. Ookla helps ISP to measure their internet speed hence it isn't that strange that most ISP's suggest you to use the Ookla Speedtest.
Last but not least it is important to know that when you perform a speed test there are some factors affecting your speed test results.