You're forking a process, not the whole system. Sorry, but I fail to see the ambiguity. If the window is in the process as some framebuffer, it gets cloned too. That may not mean you'll get two windows on your monitor, because your display engine is HW thing, and not a process.
It's because you're thinking "it does X! it's not ambiguous!" but still missing that the question is over why it SHOULD do X instead of Y, not over whether it does X or Y. To a user, it sure as heck doesn't make sense to fork a process and still end up with one window for both of them. For lots of processes the process is its windows as far as the user is concerned.
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.
