WARNING: PURE SPECULATION
I'm changing my tune. Yesterday I thought the 750,000 BTC figure was FUD to get certain Gox creditors to voluntarily accept 10 cents on the dollar for their GoxBTC. The theory I'm leaning towards now is that the 750,000 BTC figure is true and Gox has indeed been operating as a fractional reserve.
I agree this is the most plausible explanation.
There is another piece of evidence pointing in the same direction Im trying to dig up. A few weeks ago, well before Gox went down, someone posted that Mark K once said there was a "secret" that needed protecting, and if unveiled, would cause tremendous damage to bitcoin. It was not a direct quote, I cant find a direct quote, I dont recall who attributed it to Mark. Anyone remember reading anything along those lines?