It's kinda strange actually with the leaderboard. Like you said, the person with the most newsletter signups isn't guaranteed to win the Semi race each week. And the people with the most signups are the same people who get lots of referrals and already have the secret levels unlocked, so if they get 5 more referrals (highly likely), they're going to get the Semi race anyway. So why bother collecting thousands of email addresses for the newsletter, when next week, they all get reset to zero anyway?
If they were smart about it, they would spread out those signups over the course of each week and not all at once so they have lots of entries in each week's drawing. I don't suspect those people in the top 10 right now can generate that many new signups each week, unless they are spoofing the system and submitting hundreds or thousands of fake email addresses (very easy to do).