There are several common reasons why this might be the case.
Ad blocker
Browsers with an ad blocker activated are never served an ad, so they play no role in the data a user sees in their analytics tools.
Cookies
If a user clears their cookies, tracking data is lost. This might skew campaign analytics.
Google Analytics
If an advertiser turns off Google Analytics, but does not block ads, impressions and clicks would be counted by Google AdWords and iPromote, but correlating Google Analytics data would not exist.