The speeds you get from any test are best-effort scenario!
When you do an actual internet-related task, such as streaming or surfing the web, the performance will also depend on the servers of all parties involved (your service provider, the service you're streaming from, etc.). These servers may or may not be capable of delivering your content as fast as your device is capable of receiving it.
To properly conduct a speed test:
1) Close any other apps, windows, programs, downloads...etc. BEFORE testing. The computer you are testing on should be the only device on the internet at the time of testing. If you have a wireless network, make sure all phones, laptops and gaming devices are off or disconnected from the network. Often times these devices automatically connect as soon as they are powered on and will impact your speed test results.
2) Make sure the device you’re using for the speed test is wired directly to your modem (i.e. not a wireless connection). Nearby wireless networks can severely degrade the signal strength of your Wi-Fi and give you an inaccurately low reading. This is particularly common in apartment complexes and neighborhoods where there may be several wireless networks within a smaller radius.