I think that the US FED is creating a fake supply of gold, similar to how banks "create money out of nothing". The German gold bars may have been sold to someone else (read: bank loan), with Germany still paying a fee to hold their gold (read: bank account fees). But if everyone wants to get their bullion at the same time, or, in this case, if one powerful country wants to reclaim a large amount of their gold, it is not possible...because that amount of gold is simply not there at one time. Or if it is all there, they need to keep it incase they need to sell it to other people.
This keeps the market with an artificially large supply of gold, so the price can be controlled (supply and demand).