A flow bench is basically a fancy manometer. If you'd like to make one yourself, just make a U tube of acrylic that's clear, with a vacuum port on one upward leg of the U (quite a bit above fluid level.). Make gradations on the U, and fill partially with a colored fluid until centered on the gradations. From the vacuum port, run a flexible line to a test wand that's rigid and can be positioned where you'd like it. Use that same reference point for both carbs. Run a known CFM (or constant RPM for both carbs, and measure the relative readings. We don't care so much what the absolute reading is, just the relative comparison (much like a dyno.) Can also be used to synch motorcycle carbs.
A drop in pressure from IN FRONT of the carb is directly measuring the flow through the carb. If it were inside the carb, it could be affected by localized velocity differentials, but a flow bench is measuring mass flow rate, which is technically a more complicated calculation. If you were to build an airbox and put the pressure port on it, then lower pressure would have to mean more flow being pulled through. Or as Al likes to harp, "more pressure differential causing the mixture to be forced into the cylinder."