Prior to the Fed, commercial banks still held some gold as reserve in the U.S. The Fed centralized the banking gold stock during WWI. But Americans could still own gold legally until 1933-34. The statistics, however, are for gold held by the government.
The typical economic analysis uses government gold stocks or, when appropriate, the gold stock used as monetary reserves and not the total gold stocks owned by everyone in the country. Governments and banks obtain gold reserves through international trade.
Take a look at the Figure 1 World Gold Reserves 1925-1932 in this paper by Doug Irwin:
This chart gives the lie to the claim that gold flowed into the U.S. and caused the Great Depression because the Fed and Treasury sterilized the inflow. The chart clearly shows that the U.S. gold stock remained roughly the same, $4 billion, from 1925-1932. Gold did, however, flow into France. Also, the world gold reserves steadily increased from around $9 billion in 1925 to nearly $12 billion in 1932.