The clerk, enumerator, or records official may not have followed the directions they were given.
The 1950 US census instructions indicate that individuals born in a hospital outside of the state in which their parent resided at the time of their birth should be listed with a place of birth of where the parents were living.
Based on those instructions, my parents (whose parents lived in Illinois, but whose nearest hospital was in Iowa) should have had Illinois listed as their place of birth in the 1950 census. Their place of birth is listed as Iowa.
Sometimes directions are not followed.