alt Hacker NewsThe standards of purity have changed over time, and most of the US gold dates from before the 1930s; and so a lot of US-domiciled gold have different purities that don't follow the current international standard. The purity requirement has been going up over time. A lot of US gold is 90% gold and 10% copper, but gold traded on the international market needs to be at least 99.5% pure. Here [1] is a good breakdown.
[1] https://www.bficapital.com/blog/what-is-inside-fort-knox
One important detail here.
When you buy gold, you purchase based on the amount of gold, not the total amount of alloy. This is true in both markets. This difference in standard is practically not a big deal because you very rarely physically move the gold, as moving gold requires a lot of security and is a risk.