Every response to a thread also increases the number for that thread and everybody who votes down (collapses) the thread reduces it by one down to a minimum of zero. If you vote down a post, you no longer see that post. If you vote down a thread, you no longer see that thread.
The way I understand it, newbies coming in will automatically have their default for collapse set at I think 5. So if a post has 5 negative votes, the newbie will not see that post at all, at least until they change the minimums and maximums in their preferred settings. I assume they don't see threads collapsed by a lot of members either, but I'm not sure about that. That would explain why I don't think I've ever seen a newbie post on a thread with a zero rating though.