Because you've entered 1 to maximize the contribution and PTO adjusts this amount as the limitation changes.

https://proconnect.intuit.com/community/help-articles/help/sep-simple-qualified-plans/00/3488

If you actually mean IRA (rather than a self employed employer plan), does the taxpayer have an employer retirement contribution (either self employed or another employer)?  If so, the IRA deduction may change if the income is in the phase-out range.

https://www.irs.gov/retirement-plans/2019-ira-deduction-limits-effect-of-modified-agi-on-deduction-i...