I think "mrodenstei" has hit the nail on the head here; surely, the 'N' must be referring to the percentage figure (maybe it means something specific, or maybe it just means 'numerical value' etc.)
I agree, of course, that 'mois N' does often mean 'a certain reference month from which others will be counted', but since here the months are actually given, that interpretation would make no sense to me in this context.