My opinion this is because of the difficulty in withdrawing USD and generally CASH from MtGox. Leading to continious and increased flow of cash to exchange USD-BTC since it's the only way to withdraw money from there, BitStamp withdrawals are still going (for verified accounts) and therefore the flow of USD To BTC is just the normal.
So basically, a $ is worth less on MtGox due to uncertainty and difficulty in withdrawing it. Thus, traders demand more $ on MtGox in exchange for a bitcoin.
The cost of the uncertainty and difficulty can be measured by the difference between the MtGox price and the Bitstamp price, with an adjustment for fees.
Sound right?