Piece or Stück?

As I have previously written, I organize my recordings according to an old Schwann catalog (“Schwann’s Way”).  Today, however, I discovered an interesting classification discrepancy.  For most composers works that are called simply “pieces” are listed under “P” in the alphabetical ordering of title.  However, the solo piano compositions of Karlheinz Stockhausen are listed as “Klavierstücke.”  Was this an effort on the part of the Schwann editors to emphasize that Stockhausen’s works were, in some inherent way, “different?”