There has been some heavy criticism of the Perth Mint running out of retail size silver bars on the SilverStackers forum. Some good questions, some crap ones and I've tried to address them all. My first comment starts on page four of the thread and I've just posted probably my last response on page eight. Worth reading if you want to get a sense of some of the issues we face.
I'll probably rework those comments along with past posts here and here covering similar material into a more definitive post on the commercial factors the Mint has to face/consider.
Also worth reading is this series of three FT Alphaville posts on oil backwardation: I, II, and III. One would be silly to think that the same strategies aren't employed in the gold market, especially by those with the "the ability to internalise flow and keep it out of the public market (possibly within your own dark pool)".