That is used to determine if you are qualified to make a contribution in the first place. Not how much you can contribute.
If you have self-employment income you can only contribute up to your net profit reduced by the deduction allowed for the ER portion of your self-employment taxes. See IRS publication 590A (page 39 for ROTH) https://www.irs.gov/pub/irs-pdf/p590a.pdf
