The interim French investigation report explains that puzzle. Over the course of three or four minutes, Lubitz did indeed designate “100 feet” as the selected flight level. He did this several times, while the pilot was out of the cockpit.
But this was just after the plane had already begun its descent. After each occasion that he chose “100 feet” he then corrected himself and entered the correct flight level. The course of the plane was not altered at all.
The picture that builds up is of a man steeling himself for the challenge he has set himself, building up the courage but at each point pulling back – until finally the pilot re-enters the cockpit and normality returns.
Had the return flight not crashed no-one would have noticed these anomalies on the outward journey.