I have found that for me, the difficulty with nibs is a consistancy issue. I recently went through my stash of spare nibs looking for a fine one. I found absolutely no consistancy in the size.
To explain. Almost all the kit nibs I have recieved, have not been marked for size, but whenever I swaped in an upgrade I would put the kit nib in a package. I have also ordered some daycom replacement #5 nibs, specificly in fine and medium. I kept these in there own marked bags for inventory control.
In examining the swapped nibs, I ended up with three distinctive sizes. These were visually different, I did not need a micrometer to see the difference. When I checked my marked bag of fine nibs, I found two distinct size differences. In the bag of mediums I also found two sizes. When I compared all three groups together, there was four distinct sizes. This is what I mean by inconsistancy.
I did the same comparison with my herritance fine and medium nibs, and found the consistancy was much better. There was a very distinct difference between fine and medium. Among the fine nibs I could see a slight difference in a close side by side comparrison.
The same sort of visualcomparrison of my unmarked Jowo nibs from MeisterNib, I found no visual differences amongst each size.
Now I am not a master tuner. I have never been taught how to tune, but I know a lot of the theory and generaly tweek until I'm happy. In other words, it takes me quite a lot of fiddling with any nib. That said, I have made nibs from all three groups write like butter. However, I have only found untuneable nibs, in the kit catagory.
In manufacturing, there are many areas that can be adjusted for cost control. Plating thickness, nib tiping dimensions, stamping or engraving as well as many other aspects. There is also the quality control. What may be unacceptable deviation from spec for one client easily falls within the requirements of another.
This is all just my personal observations and opinions, given in the spirit of proffesional discussion. Its always nice to hear the opinons of my colleagues and peers.
Here are some good articles from Edison Pen Co.
http://edisonpen.com/articles-2