Firstly, you should make sure the payoff is recorded correctly, and doesn't have a bad mark or was accidentally recorded as late. The account should say "paid-satisfied" or something like that, if you paid the balance in full without negotiating a lower payoff. You can get your official full credit reports from www.annualcreditreport.com once a year for each bureau. You can also get reports from the bureaus by registering accounts with them, although beware of attempts to get you to agree to pricey upgrades.
Assuming it was recorded correctly, the only thing I can think of is length of credit history. One factor in your score is the length of your credit history--your oldest account and the average age of your accounts. If you closed the oldest loan, that would temporarily drop the length of your credit history.