Difference price action, volume and momentum from two exchange site that causs why the price are not the same just take a look at the buy and sell order from two exchange site look at the liquidity.
Some trader use a method of arbitrage when you buy coins in an exchange with a low price and sell to another exchange site with a high price.
This is where arbitrage opportunities do happen on where the price do have big difference between exchangers but as said these situations arent that common.
All movements and prices will vary or depend on exchange demand between into its traders so we might see different prices in the other side but they arent that much of the difference.So its rare to see these kind of chances though.If you do able to arbitrage then you are lucky enough.