Both Webtrekk and Google use a session timeout of 30 minutes. So if the user is not active on the website for more than 30 minutes (no page impression, click, etc.), any access to the website will be counted as a new visit. This period can be individually adjusted in Webtrekk.
In addition, Google automatically generates a new visit when another external entry occurs, regardless how much time has passed since the last visit. Thus, Webtrekk can measure several campaigns for one visit, whereas Google Analytics opens a new session as soon as another campaign is measured. This may result in a higher number of visits in Google Analytics.