That is a questionable interpretation. It means the 98% one lets twice as much air in, but it doesn't block twice as much wind because the amount of ambient wind remains constant.

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%.