Occasionally it happens that child support after a step-down (that is, with one fewer child) is actually higher than originally.
The reason for this odd result is that the child support amount is the difference between the parties’ obligations, not the absolute amount.
The combined obligation does go down when the stepdown occurs, but the difference between the parties’ obligations could switch in such a way that the payer ends up paying more.
This happens only in cases with substantial shared parenting.
To see this, print the stepdown guideline worksheets, from Print Guideline > Step-downs > Print the Child Support Guideline Worksheet for all step-down tiers, and follow the math through.
Here is one such case:
Two children:

One child:

Note that the basic obligation on line 10 has dropped from $3,087 to $1,982.
But after the multiplication on line 13, the difference between the parties’ obligations increases, from $1 to $4.
It’s just an effect of the formula that Florida uses.