From the IRS letter: "You entered your enrollment premiums, applicable second lowest cost silver plan (SLCSP) premium, and advance payment of PTC amounts as an annual calculation on line 11 of form 8962, Premium Tax Credit. However, you should have used the monthly calculation chart on lines 12 to 23 to compute your total premium tax credit because . . . Information reported to us from Health Insurance Marketplace shows your enrollment premiums, applicable SLCSP premium, and/or advance payment of PTC amount vary for one or more months. "
