The reason for the $5 divergence is because Mt.Gox isn't an exchange and therefore the price on Gox isn't accurate.
Last time I used Mt.Gox it was an exchange. I was able to put in buy/sell orders at prices I was willing to exchange my BTC. But at CoinBase I just have a fixed amount I can pay for BTC or sell BTC for. You can't put in a "bid" or "ask" like at an exchange. But if Gox is not accurate, who is? CoinBase has to be basing their buy/sell prices on something.
Its not Gox I can promise you that. Gox fails to meet the requirement of an exchange because you cannot reliably or efficiently exchange BTC into fiat. Sure, it might say that you have a fiat balance, but you can only realize that fiat if it is wired to your bank account.