Different exchanges are different volume, small exchange will be small volume of trading and it will violate a lot, better to trade high volume exchanges are good and there is not international system for cryptocurrency trading like forex, gold,etc..
The first thing is liquidity. Bitcoin trading volumes can be huge on large exchanges on one side and much lower on smaller exchanges on the other side. These differences in supply affect the price. Secondly no common way has so far been established to price Bitcoin, which means nobody knows what it is supposed to cost. Thus the price is based purely decided by market forces.
The third thing is that moving money across exchanges can be risky. Doing it requires a lot of information. Thus we can say it is hard for traders to simultaneously buy and sell across exchanges, which allows these price differences to persist for long.