With Webtrekk, the conversion shows how often a website target of the page has been reached. This metric depends on the configuration of the website goals in Q3. By default, the website target Order is set, but other targets can also be defined.
Google Analytics' conversion metrics show how often a website target has been reached online or offline. Examples of this could be newsletter subscriptions or orders. Here it would be important to check which conversions are compared with each other. Google Analytics also distinguishes between micro-conversions and macro-conversions. A macro conversion would be e.g. an order, a micro conversion a newsletter subscription, which is an intermediate destination on the way to a macro conversion.
So if you compare the conversions in Webtrekk with those in Google Analytics, make sure you compare the same conversion points.
The standard attribution is used in the campaign analysis for Webtrekk. This can be adapted in Q3 under Configuration > Marketing Configuration. No attribution model is used in customer journeys. So also check whether the same attribution is used in both tools.
In Webtrekk, the time of the actual conversion is specified for the conversion, i.e. if you clicked an ad yesterday and then connect directly (without campaign) to the website again today and place an order, the conversion is measured for today.
However, Google Analytics measures the time of the last ad click before the conversion, not the time of the actual conversion. In the above scenario, this would mean that your conversion would be assigned to yesterday. Here it helps if necessary only to compare the order numbers from both tools to determine whether and how the conversion was recorded, attributed and processed.
Read more about conversions in Google Analytics here.