im confused about the pines family dynamic.

i'm confused because ford and stan are the uncles of dipper and mabel who are shermie's kids, stan calls them "grandkids"

im confused cause gruncle stan calls dipper and mabel "shermie's grandkids" to ford and ford knows he is an uncle with a niece and nephew. why grandkids? is it a typo or they call them that cause of the massive age gap since shermie was born when the 2 stans were 18?