I don't understand why this is not known, either. It's being paid in some sort of cycle. Even if payroll is quarterly, the business has made payments-to-date for that pay period. Example:

Semi-monthly: Paychecks are issued on the 5th or so (for pay period from prior mid-month to end of month). That will be the first paycheck of the new year, and any health premium paid is added. Paychecks are issued around the 17th or so (for pay period from 1st to mid-month) and any health premium paid is added. It is unusual for there to be two health premium payments each month.

Every pay cycle has a similar pace.

And whoever is doing this payroll isn't done until all year end benefits and fringe and other considerations are included. You don't "add" to a W2. It's not even issued until the data is collected and then it is reported. That's why a W2 isn't due by Dec 31. And that's why payroll clerks don't run the last paycheck of the year until all taxable fringes are known, since there needs to be some sort of way to do withholding (or the employer has to provide an employee advance to cover withholding, and that gets messy).