Why is Claude Cerval so low down in the IMDb list of credits?
Well, why is he? (He doesn't even make the main list.)
After all, after the three principals (Blain, Brialy and Mayniel) he's the most important of the supporting cast.
Because the cast is given (as it says) in credits order. In the film, they appear in just that order, with Cerval after all the others. However, his name appears in the film in larger type and preceded by a French phrase similar to "and featuring", so in this case being at the end is a distinguished position of respect.