You never will, your not comparing apples with apples.
Only consistent one will be a bt test to you master socket that should remain the same. Eg 8mb
If you have an 8mb line you should expect 6mb on a good day from somewhere like Microsoft. You will get a different result from speed test company's every time as there may be more people testing that moment, or everyone in your town decided to login. It's not just about your download speed but there upload speed as well . If your router shows what your paying for give or take 10% then that's all you can do