I don’t think the math is mathing. How is this possible?

This is a question about an existing app (you don't tell which unfortunately), not for an app development. So this forum is not the right place to post the question.

is there a way to fix this?

You should ask on the app's developer forum.

For some reason, the app has registered a total screen time of 139h05'. But only a small part falling in one of the categories. Hence, an average oh 19h52' per day (math is exact).

It is up to you to understand how total screen time is accounted depending on the settings you may have defined.