That's irrelevant. The speedometer reading is an absolute reference. The percentages being discussed are simply comparisons, and they're only being discussed to say "they behave like you'd expect."
To directly answer your original question: a reading of 21 is 5% better than a reading of 20 because 21 is 5% greater than 20, and this means that a 21 speed browser should do things 5% faster than a 20 speed browser.
To... itself? Go measure something. You now have a reference!
If you scratch out the labels of your car speedometer and forget which is which, it still measures speed. 80 is still 33% faster than 60, regardless of the units.
I suspect your questions would be answered better by playing around with the tool in question for a few minutes anyway, as you seem to be asking about capabilities the tool does not purport to have.