This might be due to the fact that clicks on the Company Profile or the XING profile of the user who posted the ad were also counted, as these clicks wouldn’t have been generated without the ad. The click then leads to the Company Profile or personal profile and not to the advertised content.
Clicks are also counted whenever a user clicks on the "Interesting" button in an ad, even though these clicks cannot be traced by your tracking tool or recognised as a visit to a XING profile or a Business Page. This kind of click can be seen though by the advertiser in the reactions at the top of their personal start page. Deselecting the "Interesting" button is not shown, however, although this too also represents a click on the ad and is therefore counted too.
Clicks in the XING apps will not be recorded by your tracking tool either when you track page impressions on XING.com.