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.
But that’s not the point though. What is the _real_ difference between 98% and 99% in real world terms? How much colder will you feel? Will it dry out your skin faster? How much faster? It’s a difference against zero.
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.