Not all votes are counted, and not all votes that are counted are displayed (for instance, when you downvote someone even if they are at -4 the votecount seems to be stuck at -4 but the karma of the person is still decremented).
This question and a bunch of others I've written up in this article: http://news.ycombinator.com/item?id=1755533 .
No, then it wouldn't be secret anymore.
Security by obscurity is not perfect but in this case having that knowledge out there would only benefit the spammers and the 'gamers'.
Is news.ycombinator.com open source ?
Yes it is, the code is written in a language layered on top of scheme called 'Arc', you can get the whole package here: http://arclanguage.org/ , the package includes a slight variation (the 'secret sauce' is missing) on the code running HN.