Two-part notation of chords consisting of two or more notes is sometimes unclear in Chopin's manuscripts as regards the allocation of notes to particular voices. This results from the composer's habit of writing notes with stems always on the right side of the heads; it sometimes happens - as is the case here - that all the notes of the chord share a single stem directed both upwards and downwards.
In this case, the notation of A could be reasonably interpreted as 
. However, it seems more probable that Chopin, having already written the single-voice sixth a1-f2 changed the concept of this detail introducing b flat1 as the lower note and the two-part notation (cf. remark to the left hand). Leaving a section of the stem between the notes was probably due to Chopin's intention to avoid erasures and deletions in the manuscript. We come across the same situation in bar 13.
Cf. also Impromptu in A flat Op. 29, bar 74.
Compare the passage in the sources »
category imprint: Interpretations within context; Corrections & alterations
issues: Corrections in A
notation: Rhythm