Yes, it's possible that this is correct. The marking of box 13 depends on the plan's fiscal-year end date, so if additions were made to the plan for your benefit in 2017 but for the plan tax year ending in 2018, you are considered covered for 2018. Plan end dates are the last day of some month, so in your case, if additions were made in November 2017 and the plan has an end date of anything other than November 30 or December 31, your November 2017 additions were made for the plan year ending in 2018 (on the last day of some month January through October) and make you covered for 2018.
