Question about Health Insurance Tax Credit

Hi.  I had almost $29k in income last year, most of which was Social Security.  After some Capital Losses and the Standard deduction, the online version calculated my "Income" (I think taxible) at about $2300.  Oh, this included $200 in earned revenue and some interest.  No taxes were witheld on them.

 

I am on a Medicare Advantage Program that is an HMO.   Social Security pays them directly.  I have to pay them a few dollars each month. It shows up on the State Marketplace.

 

From, the tax tables., I thought I'd ow about $200, but TT calculated about $800.   I dug and dug and saw a note about the tax credit, but it didn't make sense to me.   I can't seem to find that page.

 

Could someone please explain why I'd be paying about 31% of my AGI?  This makes no sense to me.