BLP has an "interesting" way of flow-testing their carburetors: "Carburetors are Dry Air Flow tested at 20.4” on a SuperFlow SF-1200 FlowCom flow bench using a 2” open hole spacer."

Ever look at their flow ratings for significantly different carb sizes? Straight from their web site:
- "750" 4150 w/ 1.405" Venturi 1.688" = 1030 CFM
- "850" 4150 w/ 1.590" Venturi 1.750" = 1060 CFM
- "950" 4150 w/ 1.405" Venturi 1.750" = 1060 CFM

I suspect when you flow test carbs on a 2" open spacer that you're going to see less sensitivity to venturi & throttle size changes. If they were tested where the actual flow exiting the carburetor was isolated & measured (i.e., not dumped into a 2"-tall plenum first), the differences would be more apparent, and the max flow #s wouldn't be as high, either.