Good catch! The reason is straight forward: YouTube doesn't count a play unless the user actively clicks the play button. SubmitHub is automatic: when it changes from one song to the next, YouTube loads and starts playing automatically -- the outlet doesn't have to click anything. This is one of the ways that YouTube attempts to stop bot-farms from generating fake plays on videos.
Here is a discussion about it, including an official weigh in from YouTube themselves. Screenshots below if you don't feel like visiting the article!