I think codeforces should include a system like rated or unrated before contest otherwise sometimes it effects the rating changes.As far as I know rating changes depends on number of participants when many participants see Problem A is a bit of difficult they quit the contest which reduces the number of participants. So if codeforces can bring a system like you have to ensure yourself whether you want to be rated or unrated just before the contest then It could be more fair who participated the contest.