Because the plum color is different, it has less sheen. It also has no Off white or light bronze markings on the slide. This is a Neidersachsen Police trade in and they all had some markings on the right side of the slide. The ones that are refinished were filled over by whatever coating they...
It's part of the aging on the original bluing. As far as specifics, not sure what bluing they used or what causes it.
This particular gun(above by OP) is refinished with a fake plum to try and copy that. CDNN eith did this or had someone do this.
Here's a picture of original bluing that has age...
I got lucky on the 2000. I was picking up another gun at my LGS and happen to scan the case and they had this new 2000 for cheap. I snatched it up along with a Walther P5 Compact at very good prices. They were on consignment from an estate. Yes, it was a good day indeed.
I have one but I'm in Texas :(. Good luck bud. Here it is just in case you want to check it out.
https://texasguntrader.com/category/900/Handguns-Pistols/listings/1284151/WTS-Sphinx-AT-380.html