Supply and Demand
Bitcoin exchanges are places where people who have Bitcoin (supply) can sell it to those who want it (demand). This also means that if you have two exchanges —Exchange A and Exchange B— and both support USD and BTC there is a high possibility of an arbitrage opportunity occurring. The price of Bitcoin will not always be the same on both. The price will simply be whatever supply and demand dictates.
If an exchange has more people selling than they are buying, the price is likely to drop, as supply outweighs demand.
The opposite applies when more people are buying on that exchange, should demand outweigh supply, the price will go up.
Luno is an exchange which has until recently operated in lower volume markets outside of the US dollar-based exchanges.
These lower volume markets have displayed supply and demand behaviour that often does not correlate with the bigger exchanges, thus creating numerous arbitrage opportunities.