[roughbeast]

Speed test results aren't just dependent on your network. They are also affected by the device you are measuring speed on.

Speed tests use up PC resources. For example, a speed test will use up processor time. If your processor capacity is pretty low, then a test may max it out. Symptoms include variable speed test results and wildly varying speed during the test.

You can check this out by having your processor performance showing whilst you are running a speed test. If 90% or more of processor resources are being used that will be a likely cause of inconsistent result. You say your mobile results are more consistent. No doubt your mobile is handling the test comfortably.