I looked into this a bit further and it's related to the 2023 contribution that was made after Dec 31 + the equation of the 590b worksheet. What appears to be happening is that 8606's line 6 is calculated by omitting contributions made after Dec 31, but 590b's line 7 isn't. The effect this has is to defer taxes until next year (since having no taxable amount for this year reduces my IRA basis for next year)
