The slur is one of the improvements of notation introduced most probably in [A2] (→GE). In A1 one can see a crossing-out here; it forced Chopin to rewrite the 1st beat of the bar on the stave below. The first was most probably the version with grace note (like in b. 77), which Chopin then deleted and wrote a simultaneously played D-d octave next to it. However, the bottom note had to be rewritten for some reason; after the final form of that figure had been developed, Chopin decided to write it out neatly.
Compare the passage in the sources »
category imprint: Differences between sources; Corrections & alterations; Source & stylistic information
issues: Corrections in A
notation: Slurs