It doesn't matter if the manufacturer starts with factory barrels to cut down or not - the point for Canadian law is that the company making the firearms/barrels is legally a licensed firearms manufacturer. This is no different than how a gunsmith can cut a shotgun barrel down way under legal limit in the process of sleeving in a new barrel without it being illegal. The point of the law is that Joe Blow can't just cut down his barrel himself. They are likely looking at safety, as in thinking that if a particular firearm modification is not inherently safe, a company would not sell firearms modified in that way, because they would be opening themselves up wide to litigation.