So... in an admittedly cursory review of that form:
http://www.irs.gov/pub/irs-dft/f8962--dft.pdf
... I see a provision for REPAYMENT of excess subsidy (Part 3, Lines 27, 28 and 29), which is as-expected. You underestimated your income... you are not eligible for all the subsidy... you have to pay it back. No news here. It's fair.
HOWEVER:
I see NO provision for deficient subsidy. None. You OVERESTIMATED your income... you were eligible for MORE subsidy... you are OUT money.
So there is the answer to the question.
If you OWE them, they want it. If they owe YOU, tough and too bad.
OK... now I'm ticked.