Where did this myth come from?

For me personally, I have found that the copper washed bulk 22LR ammo is a bit "slicker" for feeding stubborn semi autos. I have not seen anything cleaner about it. However, those same picky semi autos when fed premium lead nose ammo, they run just fine. There's a reason why I bought those semi auto's in 22LR though, and it's to shoot cheap ammo, not the premium stuff. Federal 550 round copper washed bulk pack ammo runs in every semi auto bottom feeder I own. Cheap to shoot, and reliable. For me, it has nothing to do with cleanliness, it has everything to do with reliable feeding on the cheap.