Most likely because no FICA taxes were deducted from your paychecks. That's Social security taxes and some jobs don't require the payment. It's cheaper for the employer because they have to pay the same amount you do. If you want this to count on your SS, you would have to pay the entire amount. If it were me I don't think the amount would be significant in view of the way SS payments are calculated. You have a long time before you need to worry about that.
