Individual crypto exchanges may have order books consisting of differing proportions of buyers and sellers. This could lead to some platforms costing consistently more or less, to buy a cryptocurrency.
For example, at the time of writing, buying one Bitcoin costs 34.25 lakh on WazirX, while the same costs 32.8 lakh on Coinbase. However, users could sell at corresponding price levels as well. This is why the price difference can be maintained without price arbitrage by investors making it less profitable to buy cheaper Bitcoin on Coinbase and then sell it on WazirX.
Simply put, price arbitrage happens when a person buys a cryptocurrency where its cheaper and then sells it where it is costlier, to profit from the price difference. But factors such as fees and currency conversion costs end up equalising the effective price. This spread between buying and selling prices is largely down to market demand, but a sliver of profits from here go towards the exchange, as exchanges themselves hold vast amounts of coins.