I suppose it could be that until recent history, there were many more different coins in circulation around all corners of Europe, which eventually settled down into modern currency. Zloty could have been more the name of a coin than the actual currency, rather lkike the UK has a currency called the Sterling.
The first 2 letters of a currency code are always the code for the country, PL(N) for Poland, GB(P) for Greast Britain, US(D) for the United States and so on.