Probably was at the builder for the dyno tests, but likely the owner never retuned it once it was in the car.
It seems to be a common problem and separates those who really know carbureted engines and those that just buy one done already. There are a lot of folks out there that don't realize that the carb and ignition timing needs to be constantly tuned and re-tuned depending on air temp, humidity, altitude, etc...especially in a high performance engine.
I adjust my idle mixture and timing at least 3 times a year and if I was really pushing the engine I would adjust more than that...and more often too.