MtGox trades 'mtgox dollars' for 'mtgox bitcoins'. Those are not the same as real dollars and real bitcoins.

It used to be that dollars were really hard to get out of MtGox, so those dollars were worth less and this caused the bitcoin exchange rate on mtgox to seem higher than on other exchanges.

Now that GoxBitcoins are even harder to withdraw than GoxDollars, the rate has gone from a surplus to a deficit.

Just stay away from Gox, only solution.