The time it takes to verify each of the transactions (buying and selling) can add up and the exchange rate might change within that timeframe.
Many exchanges require a lot of verifications in order to trade a large number of Bitcoins.
Depositing Currency that a government has declared to be legal tender, but is not backed by a physical commodity. For example the US dollar, the Euro and almost any other country related currency you can think of. fiat currency can be a time taking process (can take up to 10 days depending on your payment method).
Many exchanges have fees, which I have overlooked in the given example, that you should take into account.
Pay attention to the transaction volume on each exchange as you may end up not being able to sell all of the Bitcoin you bought at the cheaper exchange.
Price differences also reflect an exchange’s reputation. For example, BTC-E’s exchange rate is lower these days because less people are trusting the exchange to handle their money correctly. Less trust = less buyers = lower exchange rate. The same thing happened during the last days of Mt.Gox where the price was lower and lower because people didn’t trust the exchange to allow them to withdraw their money.
