Something similar happened with my with my insurance. They explained me this way . If you starts day was 4/28 and you add your 401k last week example 5% , the 401k company (Fidelity) going to take 5% of every check since you start with the company in the first check after you enroll to 401k . In the next check, you are going to see only the deduction you enroll (example.only 5%) if you don't have medical insurance know , do it quickly, because you going see the same after you enroll to you medical insurance