I havent looked at code, so i can only guess. Just before you looked at it, i clicked downvote. I realise my mistake and unclick/click it again. The local server we're browsing on (Server A) sends the downvote to the server the post was made on(Server B), but it also temporarily adjusts it local figure for a while until it can refresh from the server the post was made on. Last it knew server B had a tally at 0 downvotes, so it goes negative. (If Server A had a downvote figure of 4, the same process would have shown 3 to you)