There are several reasons why similar Facebook ads (or campaigns) might perform differently. Some of the reasons are common to digital advertising campaigns. And some are unique to the inner workings of Facebook, which are generally outside the view of iPromote.
For example, campaigns with similar ads might serve in different geographical locations, which will account for differences in performance. Or two ads might be very similar, but not identical, and one ad creative might be more appealing to an audience than the other and therefore perform better.
Multiple campaigns with very similar ads
In the event a single advertiser has multiple campaigns running ads that are very similar, including targeting settings, you might run into a problem. Facebook flags such scenarios as unnecessary duplication, and possibly as an attempt to spam.
You should de-duplicate your ads by having distinct ads in different campaigns.
For Facebook-specific explanations, see Troubleshooting your ads.