The posthumously published edition of this impromptu was prepared by one of his students, Fontana. It is quite different to the actual autograph. Some notable differences are that it is simplified and more technically accessible (too easy, even for Chopin to write); inner voices are altered, notes are deleted, expressive markings altered, et al. Any pianist who is well-familiarized with Chopin's musical and technical oeuvre will note that the Fontana edition is not what Chopin would have written - it doesn't feel genuine nor does it sound genuine.
Most pianist play the Fontana edition because it was the first and most widely available and it is easier to play. Also, most pianists are unaware that they are using the Fontana edition as they are ignorant of any other edition.
Fontana edited most of Chopin's works that were published posthumously including his Op.69 Waltzes, which also differ from the autograph. The B minor Waltz (of this same opus) adds an inner voice which adds to the texture and actually sounds better than the autograph.
The Autogrpah score was composed for Madame la Baronne d'Este.