Because the range I bought it from does their own reloading and they told me that lot was from military surplus brass. Upon very close inspection it appears that the extraction groove on NATO brass (or at least the stuff they had) is not as deep as the off the shelf stuff. Just to be sure of the problem, we tried the ammo in multiple guns alongside mine and mine was the only one with the problem, but no problem with off the shelf stuff. They also let me try some 9mm rounds they reloaded in non-NATO brass; worked great.
Practice doesn't make perfect, it make permanent.
When I hear a Liberal say "I can speak for gun owners because I own a shotgun" I just want to scream "Well I can speak for vegetarians because I like lettuce on my cheeseburger!"