Those terms for code-review voting are (almost certainly) from Apache, which treats them differently. From http://www.apache.org/foundation/voting.html :

+0: 'I don't feel strongly about it, but I'm okay with this.'

-0: 'I won't get in the way, but I'd rather we didn't do this.'

(These votes are strings, not numbers, though they closely resemble numbers. See also the existence of a "++1" vote.)