Bold thread titles

When I view a topic list for the first time on a given day, some of the titles are bold, with the red icon on the left. Clearly, this indicates new posts since my last visit.

There are others with titles that are not bold, and the blue icon. This is supposed to mean no new posts. However, the time(s) shown for the last post is after my last visit. How can this be?

And, why don't all threads at the top have the red icon, followed by only those threads with a blue icon? Instead, I see red ones at the top followed by some blues, then more reds, etc. What's going on?