Double Reply to Thread not on New Posts

Just an observation really, unless anyone has any thoughts?

If I reply to a thread it creates a post (i.e. as you would expect)

If I reply again to that thread (and no one else has replied in between) then
a) my post is appended to my previous post
b) the date of the most recent edit to the thread does NOT show the later post's time.

I have just looked (by chance) at a post that I read earlier, and the personal has made a further reply with additional information but I was unaware of it as neither New Posts nor Today's Posts indicated that the thread had changed (i.e. they show the time of the first post, rather than the time of the later second one)

Note: I don't mean that the person edited their post (separate debate about whether that should "tickle" the New Posts list, or not), but rather they posted an additional reply - which has been concatenated with their earlier one.