Exhaust for a slant

Duals are mostly for looks. The first Dart V-8's used a single exhaust, even in the Commando HiPo version. I read that they tested a dual setup and found the single made more power. In the 70's, many speed places offered "4 into 1" exhausts for Japanese motorcycles, claiming 25% more power. When Honda later designed a 4-into-1 exhaust, the same companies started selling individual pipes, with the same claimed 25% power increase. What do you infer from that?