According to us, the presence of an additional slur over the middle R.H. voice is related to the crossings-out visible in A related to the changes of layout: the voice was originally written on the bottom stave. The crossings-out separated the c
1 minim from b, which probably prompted Chopin to enter a slur that would emphasise the course of the melodic line. This assumption is confirmed by the notation of the remaining three analogous places, which are devoid of both crossings-out and such a slur. It would be somewhat a special case of a mistake (unchecked effect) caused by a correction. Taking into account the above, in the main text we do not give that slur. It is also absent in FC: Fontana could have assumed that the slur, going through a crossed-out area, was also crossed out. Another possibility is that Chopin could have added it in A after having drawn up FC (it could also have been a common oversight). Anyway, Chopin did not add a slur upon seeing those bars in FC without one; however, he added a 
hairpin. As he added a hairpin also in the three remaining analogous places, according to us, we can assume that it was that way of drawing attention to the sequence of the middle voice that he considered most proper and hence forwent additional slurs. The slur in GE1 is a result of a mistake: the engraver misinterpreted the tie of e1. See also the note on the curved lines in b. 295-297.
Compare the passage in the sources»
category imprint: Differences between sources; Editorial revisions; Corrections & alterations
issues: Corrections in A, Errors resulting from corrections, Deletions in A
notation: Slurs