When you change the overnights from 182 to 183 overnights, the program changes ”Custody for Guidelines” (read as Majority Time Parent) from one parent to the other.
In order to keep things consistent tax-wise, it also changes the filing status from Single to Head of Household for the majority time parent, and Head of Household to Single for the other parent.
It may also be changing the tax dependency exemptions.
If you control for those tax changes, the results are the same, but you have to go back and flip those each time you change the majority time parent.
If you start with 182.5 overnights, the two parents have exactly equal time, but the child support guideline calculation requires one or the other must be the Majority time (Custodial) parent.
Also, under the tax law, neither parent would be entitled to the tax-favored Head of Household filing status, because neither would have custody for the majority of the year.
Always check and confirm the tax filing status and dependency exemptions with each scenario.
Also be aware that if there are two or more chldren, and each parent spends 183 or more overnights with one or more children, then the split custody calculation will apply.
The split custody calculation creates two guideline worksheets. One is only for the children in the custody of Party A, and the other is only for children in the custody of Party B. The software then nets the two results to give the ultimate guideline amount.