This is just... the most bizarre thing.
My cousin's kids call me "uncle." In truth, they are "first cousins, once removed."
But "First cousin, once removed Zack" is a mouthful. So since they were born, it has been "Uncle Zack."
I was confused at people acting like this was uncommon to be honest.
I had an aunt and uncle who I don't even remember the proper relation at the moment. My mom and uncles called 'them' aunt and uncle also.