Being asked to smog my car before 2 years is up?

I bought my car used last March and the seller smogged the car before selling it to me. I've recently recieved my renewal registration notice and it is asking me to smog my car again and in California you smog your car every 2 years and its only been one year and five months. So why are they asking me to smog again? Also is it okay to pay the registration renewal first before doing a smog check and is okay to send it through mail a day before the due date?