Wouter,
We have discussed WCOB in a few threads already, but to answer your question: fighters who are inactive or who fight bums score no to just a few points and thus do not move up in the ratings. Others who do fight decent opponents score more and thus overtake them rather easily. The points that can be won are linked to a place in the overall WCOB top 100 and are fixed. As far as I understood, at the start of every year they also deduct a fixed percentage of a fighter's "starting value" of the preceding year (note that WCOB works in annual cycli called a World Cup competition). This rule is exactly what you want according to your posts, as it is meant to stimulate boxers to get in the ring because they cannot rely on past perfornances forever.