Two barrels and four barrels are rated at different vacuums, 3 "hg and 1.5 "hg so two barrels are rated higher than if they were measured with the 4 bbl 1.5 "hg, higher the vacuum the more air will pass through. DV flowed the bbd at the 1.5 "hg.
A carb really don't have a static flow amount "cfm" it depends on how much vacuum is being applied/produced.