It is subjective - they feel smoother, glass-like. One factor is most cheap Chinese nibs are not cut exactly at the middle. This can be seen with a 10x+ loupe. The metal is not as hard. Different metal types have different surface tension so it affects ink flow. It works without adjustment while most Chinese nibs need adjustment, alignment and polishing (which is fun in itself to do).
The original Jinhao pen nib is quite usable, but the other parts of e.g. a x750 pen are much better, so it is a popular mod to change the nib for a German one, or a Japanese Zebra G flexible nib.
I have recently bought a Lecai Chinese pen and the nib seemed to be flawless. These guys in the right track on making a 100% Chinese upper quality pen.
Both of the sibling comments, plus good nibs don't have any rough edges which touch the paper. They've been ground/sanded down so smooth that they effectively hydroplane on the ink they're depositing.
You can improve some inexpensive nibs by simply "writing" on a high grit sandpaper or sharpening stone, but if the metal is too soft you can quickly wear away the writing surface entirely. I'd do this with the pilot disposable fountain pens with good results.
