It has to do with French government workers and socialists, if for instance it was Le Pen and Fillon in the runoff, the socialist would either go for Le Pen (not a huge likelihood but possible since she is not campaigning on cutting government) or stay home because they detest Fillon. As it appears now, Le Pen vs Macron does not put the socialists largely out of play, since they will likely vote for Macron.
That’s the rationale I’ve been seeing.
Gees, how many socialists does France have?