What makes a double pumper a double pumper?

I tried to run a street avenger on the 340 in my Duster. Actually, Street Avengers, plural. A 670 and a 770. I have an A/F gauge on that car, and while I'm not a Holley carb tuning guru an A/F gauge makes things a lot easier. Now, the 340 in my Duster is making about 400 hp, has ported heads, doesn't make a ton of vacuum at idle, and it's a 4 speed car with 3.55 rear gears which are slightly higher than recommended for my cam, 3.91's preferred. It's a daily though and 3.91's don't cut it on the freeways around here. I swapped everything in those carbs more than a few times, I worked on those things for a long time trying all kinds of different combinations. Secondary springs, power valves, pump cams, pump nozzles, jets, the works. If the a/f was decent at cruise and part throttle there was a lean spot going from part throttle to WOT. Only way to get rid of it was to go rich at part throttle (bring the power valve in early, secondaries in early, giant pump shot, etc).

Long story short, I switched to a 750 double pumper. The mechanical secondaries and second accelerator pump solved the problem with very little tuning. Going to WOT looks like flushing a toilet it dumps so much fuel, but the A/F doesn't go rich and the plugs look good so the engine must want it. I did a bunch of searching, and found a few other cases of folks with stick cars and moderately (or hotter) built engines that had a similar issue with the Street Avengers vacuum secondaries.

Like I said, I'm no carb tuning guru, so maybe I missed something. And I still plan on using the 670 SA on my Dart (auto car, milder build), everything I've seen so far says that will work great. Just my experience with my Duster.