I have had lots of friends from India and gold has always been very popular there. It was found early on that 24K or 100% (not to quibble) gold wore very quickly. Even a medallion worn around the neck would soon wear as it rubbed on the skin. The highest K gold that they found to have decent wearing power was 22K which translates to 22/24 (100) = 91.7%, close to the 90% coinage.
If I want the most accurate and factual information, before I'd trust a cereal box I'd go to a fortune teller. Everyone knows that fortune tellers are THE most trusted of all sources.
It's the flux that seperates impurities from gold. But that has a residue also, hence .999 fine. That's 99 9/10 gold or 24k. So the process is about the same. It might be noted that for taxation purposes, gold ingots smelted in the "New World" were marked at a lower purity than they actually were. Even loyal subjects were not above cheating the King, I guess ?