Comparisons between Google Analytics and Webtrekk sometimes show more or less differences. This doesn't mean, that one of the tools works incorrectly or has been poorly implemented. In fact, the processing logic of the data works differently in crucial aspects. Differences between Google Analytics and Webtrekk are specifically big for channel analyses and the attributed conversions. The most important reasons for differences are:
1. Google Analytics starts a new session with each campaign contact
Google Analytics has originally been designed as analysis tool for AdWords. This explains, that some logics are focused strongly on campaigns. In terms of session handling, Google not only works with the usual 30 minutes of inactivity prior to closing the session. It also closes a session, as soon as a new campaign contact was made. As a consequence, a new session is opened.
As Webtrekk solely works based on activity and allows multiple campaign contacts per session (user leaves the site, but returns within some minutes), this is a major reason for differing figures between Google Analytics and Webtrekk. This specifically affects traffic analyses.
2. Google Analytics cannibalizes direct accesses
A logic relevant for marketing analyses refers to the cannibalization of direct accesses. In Webtrekk each new session, which does not start with a mediacode (thus, a campaign) or a referring page, is depicted as direct access. Common cases are the direct entries of the URL, the use of bookmarks or newsletter, which have not been tagged with a mediacode. Google Analytics recognizeds direct accesses the same way.
Additionally, Google Analytics checks, whether a campaign or a referrer was tracked in the previous session. In these cases, the direct access is not depicted as direct access, but the session is assigned to the previous referrer or campaign. Practically, this is "tracked" again. This leads to the situation, that the share of direct accesses in Google Analytics is comparably low. Webtrekk doesn't use such a recourse, but always assigns the session to the valid channel. As a result, the share of direct accesses in Webtrekk is often significantly bigger.
Particularly channels with much traffic such as Organic Search or AdWords benefit from this cannibalization logic. Obviously, this has big effects on the conversions, that are attributed to these channels or direct.
This logic can be read in the Google Analytics Help at https://support.google.com/analytics/answer/6205762?hl=en. There are also a number of bloggers who have studied this behavior in detail. An example of this can be found at: https://www.e-nor.com/blog/google-analytics/direct-visits-google-analytics-attribution-precedence.