I worked in both NJ & MA last year for the same company. When I add the total wages shown on my W2 for NJ plus MA (Box 16) the amount is different then the Fed wage in box 1. Why & How?

It's because your taxable wages are different as determine for federal purposes as opposed to state purposes. For example, if your employer is deducting money from your pay check for medical or dental insurance premiums, the amount you pay is not included in our federal wages (Box 1), but is included in your NJ state wages (box 16).