As a big trading country and a country with a trade surplus, a large amount of funds will naturally flow into China, including funds in the form of gold. In addition, there are many Japanese enterprises and nationals involved in gold trading, and the trading volume of gold futures is large, so the gold in the international gold market will flow to Japan.
Another point is that gold trading was relatively free decades ago. At that time, Japan and developed countries in Europe and America had a great demand for gold, and a large amount of gold flowed into these countries internationally. Later, due to the gradual reduction of gold mines, countries began to limit the outflow of gold, so Japan accumulated a large amount of gold at that time, which also affected the current gold reserves. However, China, Russian and Indian gold-producing countries, at that time, had less gold trading volume and more gold exports, and the demand for gold only soared in recent ten years. What's more, the output of these countries in the world is not as absolute as possible. South Africa is the largest gold producer in the world, and its output is much higher than that of China and Russian.
Finally, although the gold reserves of gold-producing countries are not absolutely ahead, most of them are still relatively ahead.