I’ve just checked on the desktop (where everything seems easier) and the arrow just shows replies/quotations, and the bell shows any new post in a watched/tracked category/tag/topic. Your reply to me here only appeared in the arrow section.
My confusion maybe arise because the bell section shows one username or “x replies” (x>1). Perhaps if it said “x posts” I’d have understood all right.
I’ve just checked the tooltip text. There are no tooltips for the new vertical list of icons on the right. But the ones on the left of each notification item have tooltips. The tooltip for the arrow is “new reply”. The tooltip for the bell is “new post”. This distinction backs up my observation in the paragraph above 