This is likely due to differences in demand for BTC among different countries and regions, depending on the population of traders using each particular exchange. The differences in BTC value should not be too great, however, or exist for a long time because eventually people will see an arbitrage opportunity developing and take advantage of it, after which the price gaps between exchanges should decrease. Some small differences in BTC prices may remain simply due to normal trading activity and the cost/hassle of moving currencies between exchanges to take advantage of arbitrage opportunities.