So they're supposed to mark the new comments that we're created less the 10 mins ago. I'm not sure why you're seeing the other comment highlighted when it's a few hours old, did you follow a link to that specific comment? If so, that comment will also be highlighted
I didn't, I just noticed that 3 hour-old comments were highlighted, while 4 hour-old comments weren't (but I just saw a highlighted 4 hour-old comment and an unhighlighted 3 hour-old comment, so it might be something else).