Retirement tax questions

I don't understand this answer.  Nothing has been deducted yet.  Shouldn't it use the $10,000 as a deduction instead of subtracting the overage from your deductions?  My total state and property taxes is $21,120 and they're subtracting $11,066.  Why wouldn't it just add the $10,000 maximum to my deductions?

 

SryaninCalif