Como es que las monedas de 10 Groszy van de 1990-2016?

La unidad monetaria (el Fourth Zloty va del 1995 hasta ahora). Como es que se emitieron 5 años antes?

Será mejor que llames la atención del revisor de Polonia @Hibernia 

A good question. 

The currency was reformed in 1990: 1 new Zl = 10,000 old Zl. New coins were designed, as well as new banknotes. The new 1990 series of banknotes were found to be unsuitable and were never issued.

Existing Zloty notes continued in circulation at their new value [for example, 100,000 zl note had a purchase power of 10 new Zl]. 

This kind of duality also happened when the Euro was introduced, with pricing in national currencies and Euro equivalent until a certain date.

Shouldn't we say that the fourth Zloty was from 1990 onwards?

Probably, as there were coins minted dated 1990.