So, i was watching this video, and even if I don't speak a word of japanese neither I can read it, I've tried to translate some of the comments.
What I understand is that probably SM necrozma did not lose all the light like USUM necrozma and that's why the Z-rings are different.
Here's the video, for someone who understand japanese and is interested
https://www.youtube.com/watch?v=rzoY2WOoM6E