There’s a lot of long term sickness that is skewing the average. For instance cancer or a herniated disk can keep you from working for pretty long.
The source of the 18.4 days has a lot of further information, albeit in German: https://www.iwd.de/artikel/krankenstand-in-deutschland-49865...
What? Do I understand correctly that "Google says" and you find credible that on average a person takes 18.4 sick days every year? That sounds high by at least an order of magnitude to me. Even if I took a day off every time I had a slight sore throat or a moderate hangover, I still wouldn't take more than 3-4 sick days a year most years.