The answer is simpler than that. The issue has nothing to do with Bitcoins.

This is an issue with USD (as dollar bills in your pocket or in a bank account) and MTGXUSD (as dollar units in a Mt Gox account). MTGXUSD are worth less than USD because of two factors; one is potential regulatory issues, and the other is that you are buying future dollars, which are cheaper than right now dollars.

Because MTGXUSD are cheaper than USD, you need more of them to buy BTC, and thus the price differential.