It's a week to week ranking more or less, Niagara went undefeated this past week, on a five game win streak and have won seven of their last ten.
London for instance had just one game and it was against a lowly Guelph team, they went 1-0, 1 win is less impressive then three. Teams like London, Windsor, Erie, Kitchener, Kingston etc wont drop to far for one loss because of their overall record, but they will drop some. If this is overall for the full six weeks of the season Niagara would not be third.