Why does my zinc nut not fit on my galvanised fastener?

The coating on galvanised fasteners make it difficult for zinc or any type of non galvanised product like stainless to fit. The reason only galvanised nuts fit is because the nuts are tapped oversize to accommodate for the added thickness from the galvanising process on the bolts.  

The same issue arises when using a galvanised nut on a zinc or similair bolt, because the nut is tapped oversized, it will wind on very loosely, even creating the impression it isnt the right nut.    

Always make sure whenever ordering galvanised product, match with other galvanised products and the same can also be said whenever ordering zinc products, match with zinc (or even stainless) products.