The Child and Dependent Care credit is a percentage of your work-related expenses. However, the amount of work-related expenses that you use to calculate the credit cannot be more than the earned income of whichever spouse has the lower earned income. Why? Because that's the way Congress wrote the law.
The following is from IRS Publication 503, page 11.
Earned Income Limit
The amount of work-related expenses you use to figure
your credit can't be more than:
1. Your earned income for the year if you are single at
the end of the year, or
2. The smaller of your or your spouse's earned income
for the year if you are married at the end of the year.
