One reason clicks don't match is that the website referrer (referrer) is not communicated over HTTPS, and iPromote only uses HTTPS for its ad serving.
So when an ad is clicked, iPromote records the click. But when the applicable landing page opens, and the third-party analytics tool records data, there is no record of where the user came from and so no attribution is possible. A conversion is not counted as a conversion by third-party analytics tools because these tools never "see" the referring page.
At this time, there is no known fix for this issue. However, knowing that such discrepancies are a part of web analytics should help you keep better track of click attributions.