read changing to unread

[me just saying]

I tend to change posting from unread to read when I am finished reading the ones I am interested in. but why do they change back to unread when someone replies. for example, one thread had 185 unread posts, I changed them to read using the mark forums read link. one person replies, and now it says 186 unread posts.

Is this a bug or just the way this forum works though of all the forums I use, this is the only one that does this. this is on a pc using firefox and linux mint.