Yeah, your last guess is right. Your score is just a quick reference guide for a lender to see how risky it is to lend you money. If you've been borrowing money and paying people back, you're low risk, so you'll have a higher score. If you've been borrowing money and not paying it back, you're obviously high risk and will have a low score. But if you have no history of borrowing money, you're an unknown. And unknown is risky. Thus low score. Still, worth checking your report to see if anyone has been using your identity to borrow money irresponsibly, but I wouldn't be surprised if you just found it to be empty with no errors. 

Your score is reflecting your limited credit. As the previous person stated, lenders see the score and report and determine you don't have any credit history. Lenders want a history so they can better decide if they are OK with risking losing money by lending to you. 

If you want it to improve, apply for one trade line (credit card/line of credit) Pay as agreed. Never close this trade line. Creditors want to see you have financial responsibility over time.