On your 1095-A, if column A is the same for all 12 months and column B is the same for all 12 months, filling out Line 11 of Form 8962 is correct, and filling out Lines 12-23 is wrong.
The fact the IRS letter includes "and/or advance payment of PTC amount vary" is against what the Instructions say.
If the column A is the same for all 12 months and column B is the same for all 12 months, I would respond to the IRS notice by printing out the instructions, highlight the pertinent information on pages 12 and 13, and include a copy of your 1095-A.
https://www.irs.gov/pub/irs-pdf/i8962.pdf#page=12
I would highlight the following on pages 12 and 13 of the Instructions for Form 8962:
Check the “Yes” box and continue to line 11 if all of the following apply for each qualified health plan you or a member of your tax family was enrolled in for 2018. Otherwise, check the “No” box and continue to lines 12–23.
•You were enrolled in the qualified health plan for all 12 months during 2018.
•Your enrollment premium was the same for every month of 2018. Your enrollment premium is reported in Part III, column A, lines 21 through 32, of Form 1095-A.
•Your SLCSP premium is the same for every month of 2018. Your SLCSP premium is reported in Part III, column B, lines 21 through 32, of Form 1095-A. But see Missing or incorrect SLCSP premium on Form 1095-A
next.
and
Line 11—Annual Totals
Note. If you checked the “Yes” box on line 10 and you are completing line 11, do not complete lines 12 through 23. Once you complete line 11, skip to line 24
