What happend, is the same people who put the games together, realize that making the hardware for carts were not that hard. So many types of  "homebrews" were made and sold. Then of course China being the manifacture of many game products, have their "hongkong pirates", and in the west their was unlicensed games being sold for the NES. The NES was truly the VHS of it's day. Man I wish I could go back in time to those days, when playing Super Mario and DuckHunt was as good as it could get.

So if you do find such carts it was most likely, one of those.
Some of those games did make it into the market place alongside Nintendo licensed games.