When I read your post, I had a flood of thoughts:
The modern gold coins are .999 pure gold (as long as they are real). Older gold coins like you might stumble across in historic placer mining areas may not be as pure, depending on the age and mint that produced them. If you machine rings high on a gold coin, no matter the purity, it would be a great day to dig one up. I am still waiting for my first one.
Jewelry is all over the map. 24Kt would be pure. 14K is about half, with the other half typically copper and nickel. Jewelers do this for colour (more copper - more red hue), strength (pure gold is soft and jewelry can damage or bend), culture (eg. in India and some Arab countries, they insist on 22Kt), economics (gold is more expensive than copper), etc. When detecting jewelry, the content, shape (including width) and depth all come into play. A heavy men's ring under about 6 inches, regardless of content will give a good bang on almost any detector but may not be a really high number. The numbers will lower if it is deeper, thinner (lady's ring) or a different shape (eg. a crushed ring). In fact, it can also be different if the ring is on its side or upright.
Each detector and sometimes coil will give different numbers on the same target. It may change with coil size, shape (round vs elliptical) or type (mono vs DD). It matters if you are over the coil's sweet spot or beside it. Then you can also get masking from other nearby objects or even mineralization.
Bottom line - the numbers you get are a hint. It reminds me of The Curse of the Black Pearl movie quote: "... And thirdly, the code is more what you'd call "guidelines" than actual rules."
If you want to pay attention to the numbers, perhaps ignore iron (low) and dig everything else. The detector tells you something is there and tries to guess what it might be. It is not infallible.
