I am having similiar issues am trying to track it down. It seems that weights in campaigns are effective but weights to indivdual banner are not being calculated into probability currently and I am fairly confident that they were in earlier versions. I have been using OpenX open source for at least 5 years, though in relatively simple way, Always remenant. I was also under the impression that probabilities and weighting were a simple function of weight value of 1 banner over total weighting values. So if had Advertiser A with a single campaign (weighted 2) and a single banner for that campaign (weighted 2) and Advertiser B with a single campaign (weighted 1) and a single banner for that campaign (weighted 1) there would a be total weighting of 5, which banner from Advertiser A would be weighted 4, which should translate to 80% probability (assuming both remnant in single zone).
I have about 10 advertisers, each remnant, each with a single active campaign with 2 or 3 different sized banners, each banner linked to a single zone. I was try various combinations of weighting per campaign and weighting per banner. It seems while the combine weighting (banner * campaign) is being calculated, it is NOT being translated into correct probability, as shown in image below. The first advertiser has campaign weight of 2 and a banner weight of 3, for a total weight of 6. The second advertiser has a campaign weight of 1 and a banner weight of 1 for a total of 1. Yet the probabilities are just 2:1 ratio.
Am I misunderstanding how weighting works?