The modifications needed to AGI for the purpose of determining eligibility to contribute to a Roth IRA are stipulated in section 408A(c)(3)(B)(i) of the tax code and are listed on in Worksheet 2-1 of IRS Pub 590-A:
https://www.irs.gov/publications/p590a#en_US_2021_publink100025085
Different modifications for different purposes is odd? No. The reasons one needs to modify AGI for different purposes are dependent on the purpose for which the AGI is being used.
