Aside from the occasional hung-over or cranky monk, there are other reasons why a node might be downvoted. I find that when I click on a vote, the radio buttons remain active and will respond to my navigation keys. If I use Page Down or an arrow key, the vote may change. It could be something as simple as this - that someone clicked on an upvote, then used a key that changed the vote (and didn't notice the change).

Remember that reputation votes are simply a measure of how popular a node is. Ideally this would be based on the node's accuracy, whether it contains interesting content, is on-topic, well-written, and so on. Overall, it will have some connection to these qualities. But everyone has a different take on things, and what one person finds useful, another might find annoying or trivial. The system isn't perfect; it's just the best we've been able to come up with here.