Medical expenses worksheet line 2b - "net of adjustments" - where does this come from

Married filing jointly, retired and participating in Affordable Care Act (ACA).

2023 was first year 2024 is second year on program. I trust TurboTax, but my income will be unexpectedly changing for 2024 (from my initial ACA Estimate) so was looking into how all this reconciles -  for my 2024 tax filing expectations. I understand that part now, but through the search process (looking at 2023 TTax again) I discovered I have a question about where this  line 2b"net of adjustments" - figure comes from??? I don't see it in any "math" reviewing my 1095A totals (e.g.; C-A, B-A). It's order of magnitude close, but those 1095 math calcs are closer to $4000, and line 2b is closer to $3000

 

Any suggestions on there this specific line2b figure is coming from?