With such volatility, all exchanges vary a great deal. At time of writing this answer, bitcoin price had jumped another 20% in less than a day and it wasn't too strange to see any two exchanges vary by $500 or more (Current source). Each exchange calculates their buy and sell prices a little differently.
True exchanges like Bitfinex all have different user bases in addition to variation in their buy/sell calculations. This means they may have slightly different supply and demand effects. None of the exchanges are centrally monitored, as with stock exchanges, so they must calculate buy and sell rates on their own.
Coinbase is not a true exchange. Your buy and sell orders are not filled by others buying and selling. Instead, you are buying and selling from Coinbase. Like any currency exchange you might find at banks or international airports, Coinbase offers a larger spread (the difference between buy and sell price) as part of their profit scheme. For the user, Coinbase offers value over true exchanges by filling any order immediately and simplifying the process.