…whenever I log in, all the threads that I looked at completely say that the amount of new comments in said thread is equal to the number of posts in that thread?
'Cause the Vanilla forum software is buggy?
But seriously, I think it has to do with hitting the Mark All Viewed button. Any new post then makes the forum list every post in that discussion as unviewed. Or, at least that’s my guess, since when I hit it one time after being away for a while, it started highlighting everything as unviewed when every there was any new post in a thread that I had not looked at since hitting the button.