I have 8 coop ads that have all been enabled for over a week and that are all set to 1 (ie 12.5% each). Yet the stats show a massive difference between how often they are shown. One is in the thousands of impressions a day and one is in single figures! Anyone have any idea why?
Because they are assigned to sites for a few days. If you are assigned to a low traffic site, you get fewer impressions.
What Yfs1 says. It's not a real-time mechanism like AdSense, it's all cahced. The distribution is done by calculating the odds of a particular ad being put in a site's cache. That's based on your weight and on the per-ad weight you assign to it. That's why yuo might find discrepancies but across the board and over time the stats will normalize pretty near to what you deserve/assigned to it.