I appreciate everyone's feedback.

The background is very simple, the client lived in NJ for the entire year and the employer was located in New York for the entire year. (The client didn't work at the employer for the entire year, but that fact shouldn't have any impact on the Box 1 and Box 16 difference discovered on the NJ W-2.)

In any event, I think the only way I'd be able to get to the bottom of this is to actually contact the employer but given the time that's passed since the client worked there, doing so is not feasible. Perhaps this will be one of those mysteries that will remain so. Thanks again.