I noticed I had 6 notifications (!) and went in to look at them - 5 for the same thread, which seemed a little odd. Clicked on the bottom one, which took me to the last @ of my username, and then when I went back into notifications, they'd all been marked as read.
Posted again on that same thread - someone replied and Bingo! Another 5 notifications! all at one minute intervals. Went back - only one post @ my username on the thread. Back into notifications - all marked as read.
I know it's not a major issue (although slightly dashing to my ego! 
) but thought you should know.
Laptop, Chrome (Windows 7 - sssh!)