Probably because you tipped more than expected. Tips in Germany are usually 5–10 % and not mandatory. Traditionally the bill was rounded to the next whole number, and the tip was the difference.

[–] meowMix2525@lemm.ee 1 points 11 months ago

Interesting, that's certainly possible. But why would she think she did something wrong if I tipped more than she expected?

Tbf I'm not used to waitstaff commenting at all on tips, it was weird to me just that she watched me press the number on the card reader and then said something about it.