TurboTax is apparently using Worksheet 1-1 from IRS Pub 590-B (Taxable IRA Distribution Worksheet in TurboTax) to do the calculation. You said that your 2020 contribution was made all in 2020, so the worksheet should produce the same result as doing the calculation entirely on Form 8606. You'll need to view the Taxable IRA Distribution Worksheet in TurboTax.
