Secondly, the fact that the performance with filtering converges for such a low value of suggests that there is little true topical overlap between the newsgroups, and the fact that the performance is significantly better for suggests that there is some overlap, possibly created by a sub-topic of one of the newsgroups.
although there are only three newsgroups, four centroids are better at finding them than three centroids, because the fourth centroid may correspond to an overlap region between two clusters, which then gets assigned to the correct newsgroup.