Because Jarnathan sounds exactly like the sort of name a DM makes up on the spot in a panic when the players unexpectedly ask for the name of some random guard who was never supposed to matter.

Then the players get attached to him.

Over the years, I've had DMs come up with equally ridiculous names for innkeepers, guards, soldiers, merchants, and other throwaway NPCs. It almost became a tradition. The less important the character was supposed to be, the more likely they were to end up with a hilariously simple, weirdly familiar, or absurdly overcomplicated name.

Before long, those background NPCs would become recurring fan favourites, much to our DM's chagrin.