It doesn't take a lot to figure this smooth gun thing out.
As any target shooter will tell you, consistency is all important. If you change one thing your group will vary.
So with a smooth bore, if one small piece of crud stays lodged in the bore, your group will change. If your patch thickness, or material varies, your group will change.
If your gas pressure changes in the barrel, you group will change.
Almost everything you alter in the loading process will alter your point of impact.
That is why those guys that shoot phenomenal groups with smooth bores or rifled barrel are able to do it.
They load the same way without even thinking about it.
Old Ford