They're machined correctly, they aren't zero gap rings. Place them loosely on a scope or boring bar and you'll see they are made that way. I've mounted literally multiple hundreds of scopes, NF rings are some of my favorites. Never an issue. Just mounted some Seekins on a Havak, those were nice as well.
Leupold Backcountry's used to be decent, now with the t-27 head, they're garbage. I was in favor of the change from the 1/2" nut of the previous rings, until I mounted a few sets. They won't stay parallel to each other and end up binding the tube.