There is a YouTube bug where the videos show only thumbs ups (likes) and not thumbs downs (dislikes). It seems that there is this 15 second delay from when the video starts to when it shows the number of dislikes on a specific video.
For example, this popular Google Glass How To video initially shows 6,961 likes and 0 dislikes, then 15 seconds later it shows 6,515 likes and 448 dislikes.
Google's YouTube help representative, Keane, said in a YouTube Help thread that this is a known bug. He wrote:
We have been aware of an issue right now that causes every video to have 0 dislikes on the video page, though other parts of the site show another number.
Your likes/dislikes are still recorded as normal (go to youtube.com/my_videos or Analytics to check the accurate count). We are investigating the issue and hope to have it resolved soon.
Here is a video of this bug, please feel free to dislike the video, it won't matter right now anyway. ;-)
Forum discussion at YouTube Help.