The reason for this is because double pumpers are jetted richer than vacuum secondary carbs. As a result, the PVCRs are smaller. You can jet the double pumper down to get better mileage at cruise/steady state, but then it'll run lean at WOT. To fix the air/fuel at cruise and at WOT you have to jet the carb down and drill out the PVCRs.
It is the calibration that Holley ships the carbs with that makes them less efficient (they are after all, racing carburetors). Not the mechanical secondary itself.