I believe the thinking is because sync isn't instant, especially on older slower hard drives, having to type it again give it time to actually complete.
Back in the day (old Unix), the sync call would return right away, and the kernel would sync in the background. Unless there was a current background sync happening -- then sync would block until the first one finished, which is why you would have two sync's in a row. The third sync was thrown in just for luck.
