A brand had two models: one that blocked 98% of the wind and another that blocked 99%. To a casual observer, it wouldn’t be immediately obvious why the second one was significantly more expensive. After all, it’s just a 1% difference!
(Of course, the answer is obvious: the second one was twice as effective at blocking the wind.)
To be more explicit, if there are 100 units of wind and one blocks 98 units, the other blocks 99 units. So one lets twice as much air in than the other, but I wouldn't consider that saying that one was 'twice as effective'. The relative improvement is (99-98)/98 ~ 1%.
If windbreaker A let's in 2 units of wind out of 100 and windbreaker B lets in 1 unit of wind out of 100, that means windbreaker B is twice as effective at blocking wind, because it lets in half as much wind as windbreaker A.