An applied Character Style would be one explanation…
… and a nested style could also cause a different font.
Thank you Willi and Laubender;
turns out it was a character style but I can't figure out how that style came into play...
Is that a feature or result of something else?
When nothing is selected and you click on a style (Character Style and/or Paragraph Style) it becomes the default choice of style. When you later start writng these styles are applied automaticaly.