Schumann II

Listening for this class: Robert Schumann (1810-1856):
Davidsbündlertänze, op. 6 (1837)
Carnaval, op. 9 (1834-35)
Etudes en forme de variations (Symphonic Etudes) , op. 13 (1834)

Make sure you have read in Todd: Anthony Newcomb's article on Schumann, 258-315.
 
Why does Davidsbündlertänze have an earlier opus number but a later composition date than the other pieces here?