[anonporridge]

It looks like an archaic reason, https://www.bullionstar.com/blogs/jp-koning/golds-official-p...

tldr; the Fed owns gold certificates which give it a claim on a $11 billion worth of gold held by the US Treasury. They don't have a claim on a number of ounces of gold, so the US hasn't changed the convertibility value of gold certificates of $42.22 set in 1973 so that the Fed's claim on an absolute mass of gold remains unchanged.

It still doesn't quite make sense to me, because if the official value matched the market value, the Fed would still have the same claim to their book value of gold. It makes me think that something else is going on that's intentionally obscured.