Usually because you've had the browser open for a while and are seeing the state after the parent, but before the sibling comment was posted. At least that's usually why it happens for me; refreshing to see if someone else already has said what you want to say is rarely uppermost in my mind when a reply comes to mind.

Another possibility (pretty clearly not the case here) is when there are lots of replies: either you just miss it among the others, or ypu just can't be bothered to scroll down half a mile.