When this happens look at the top of the thread to see if there are other people surfing the same thread.
The way this works is that when the page is loaded into your browser it shows where the nods are at for all the posts, but when you push the button it updates that tally to show you "live" where the number is at (AJAX). If there are others following the same thread, and if they've nodded before you then when you click your nod and it updates to the live version it shows you the new live status (including the nods of anyone who clicked before you).
There are some very active threads today, and on topics that people are clearly passionate about. So the nods feature is being used a lot. This should only happen if there are several people reading and nodding all at the same time though. If you nod and it's not right and there's nobody else on that thread then please let me know because that would signal that the feature is broken.