there are currently three hypotheses to explain the chemical composition of 96p/machholz. one hypothesis for the difference is that 96p/machholz was an interstellar comet from outside the solar system and was captured by the sun. other possibilities are that it formed in an extremely cold region of the solar system (such that most carbon gets trapped in other molecules). given how close it approaches the sun at perihelion, repeated baking by the sun may have stripped most of its cyanogen.