The only problem is when they are not compared at the same depression with the same method & conditions. If You can get 850 thru' a 250 I wanna see it.....and hear it...
All 4V carbs are tested at 20.4 inches of water. IIRC that is 3 inches of Mercury. 2V carbs are tested at about 10 inches of water which is 1.5 inches of Mercury.
You can 850 CFM through a 250 CFM rated carb by running the test pressure up.
The biggest mistake in running a bigger carb than the math calls for is not getting the booster correct. You can run a much bigger carb if the booster is carefully selected and not lose a single HP down low and gain from the middle all the way up.