My only guess, and a wild-assed one at that, is from engine masters air cleaner test. They showed that direction of airflow can be important. They took a filter setup with a three inch element and a filter lid, taped off the three inch element, and made more power thru the lid only, cause the airflow was down into the carb, not across. They found the same with their "salad bowl" with the filter lid, and an old style velocity stack.
Maybe the scoop directs the air better/different without the filters?
