21 is apparently better than 20, but how much better? You could say "1 better", tautologically, but how does that relate to the real world?
Driving a car 1 mile per hour faster may be better, in a sense, but even if you drove 24 hours straight, it would only gain you 24 total miles, which is almost negligible on such a long trip. Nobody would be impressed by that difference.
A 5% raise for someone who makes $20k per year is $1k, whereas a 5% raise for someone who makes $200k is $10k, which would be a 50% raise for the former.
> "The score is a rescaled version of inverse time" is the key here.
> If you run all the tests in half the time, your Speedometer score will double. If your score improves by 1%, it implies that you are 1% faster on the subtests.
> (There are probably some subtleties here because we're using the geometric mean to avoid putting too much weight on any individual subtest, but the rough intuition should still hold.)
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.
TL;DR: They behave like you'd expect.
To what?
I talked about driving a car. Miles and hours are an absolute reference. We still have no absolute reference for Speedometer.
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.