Bounces in Webtrekk are the number of visits in which only one page was called. I.e. the page impressions = 1 . The Bouncerate % of Webtrekk shows how many percent of the entries have not called another page in the visit. The formula for Bouncerate % is Bounces / Entries * 100.
In Google Analytics a Bounce is counted as a session in which only a single request to the analytics server is triggered. Depending on the setup, another interaction can also consist of one event and not of a second page call. The formula for the elimination rate % is Bounces / Visits * 100.
Fundamental differences in the visit calculation can also lead to discrepancies. More information can be found in our FAQ What is the difference between visits / sessions in Webtrekk and Google Analytics?