How codechef judge the difficulty level of problem given in a contest

I am curios about this , in DEC 2014 first four questions were consider as Easy , cakewalk or simple but if you see the rate of submission of 100 points where not indicating the same. Through out the contest people felt that contest was tough but editorial and its difficulty level was stating other way. I am not in complain mood but some time it becomes very tricky to judge you own capability , so just wanted to know what takes to decide the difficulty level ?

when problem setters submit the problem they are asked to tag the question with difficulty level , then codechef verifies it whether the tag is right or not . and thus the editorialists , put the editorials with tht tag .