The marking of box 13 Retirement plan on your W-2 is quite possibly correct. It's possible that the employer made additions to your account in 2018 (say, from forfeitures from employees who separated from service before being fully vested in company matching contributions) or it's possible that additions were made to your account in 2017 but for the plan year ending in 2018 due to the plan using a fiscal year instead of a calendar year. The IRS goes by the marking of box 13, so you must enter the W-2 as received.
