Ring stamped with 18k is actually 14k?

Just an update to this post: https://www.reddit.com/r/Ebay/s/kkZgvss2xp

I finally got to a jewelry shop in the area because I badly needed to resize the ring. Didn't end up getting it resized because holy shit that's expensive, but I got some clarity on the metal purity!

They did a scratch test, at first she scratched the ring and used a solution that I guess is for 18k gold? And the mark the ring left QUICKLY vanished. The sales associate brought back another solution for 14k gold, repeated the test, and this time the streak stayed! She explained that what could have happened was that the ring is 14k with 18k plating and that while a metal purity stamp is required it doesn't have to be accurate ... Which I still don't understand ha.

I've seen a couple posts on here and seen that eBay Authentication will sometimes fail a jewelry piece because it was listed for at a certain metal purity level but it came SLIGHTLY short. Like, listed for 18k but came up 16 or 17k .... So I wonder if it failed the 18k solution test because it might be closer to 15-17k? But now I have questions about the eBay Authentication process because wtf? eBay says it's 18k, this jewelry shop says it's 14k .... What?! I guess always double check your jewelry even if eBay authenticated it?

Because I couldn't get it resized at this location, I'll be going to another shop sometime and maybe I can ask them to test it or something. Because I still feel WEARY af.

Anyways, if anyone has a possible explanation, I would love to hear it!