The 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