What it may mean is the Stelling/Helling air-filter assemblies may have been less of a restriction than the filters you replaced, thus leaning out the A/F mixture and increasing idle rpm. If this is the case, you should notice an obvious increase in power and fuel mileage.
Another possibility is the change in airflow through the filter assemblies may have improved fuel atomization and vaporization through the carburetors and in the intake runners, resulting in improved combustion. Either way, pressure drop or airflow change, you found an improvement. This being the case, think about using K&N filters.
By the way, alway tune carburetors with the complete induction system in place, which includes the air cleaners. And, no, you're not a moron. Induction systems have confounded the best fuel-systems people at one time or another. Regardless, there's always a simple reason why things happen that don't seem so simple at the time.
Good luck.