This is interesting. I agree it is most consistent with Gox not having enough USD. It costs about 3% to convert to USD at retail in Japan.
The fee is 5% plus some, so it appears there is also an international wire transfer involved, with the various fees along the way.
After their USD losses perhaps they are running a fractional reserve in USD. The real solution here would be for them to actually explain what is going on.
I think it is public knowledge that $5 million was confiscated from Mt.Gox customer funds. Considering that it is a Japanese company that does not likely have such a sum of their own, excess funds, in foreign currency, this resulted in a deficit and fractional reserve in customers' accounts. It is almost certainly the root cause of this bank run.
Could you explain where MtGox's 10 million USD profit so far this year has gone, or are you only looking at one side of the equation? Not to mention there are > 12 million USD worth of bids in the order book. 5 million doesn't explain anything. You need at least 20 million USD to explain this as a lack of funds.