If they cut the mag well with a broach, they would have had to cut the grooves full length. If they cut the well some other way, or used a different broach, they would have had to cut the grooves in a later step using a rotary mill. Just a difference in how the tooling was set up. The U.S. allowed manufacturers a fairly wide latitude in setting up their tooling as long as the product was IAW specifications.

Jim