The problem with C+ fares is that they are only bookable on Delta metal over the Atlantic as AF/KL does not have an equivalent booking class (Economy Comfort on KL is not a separate fare class). This means you can't book itins that involve LAX-AMS or LAX-CDG codeshare flights with C+ fares. These flights can only be booked as Economy, Premium Select, and D1 fares. With fewer flight options available for booking, this will generally translate into higher fares.