The time signature is most probably an error or FE's and GE's revision. Changes of time signature indication of that kind are to be found on a number of occasions also in other Chopin's pieces issued by those publishing houses – cf. e.g. Etude in C, Op. 10 No. 1, bar 1..
Compare the passage in the sources »
category imprint: Differences between sources
issues: Changes of metre, Inaccuracies in GE, Inaccuracies in FE, 4/4 or 2/2
notation: Rhythm