Why is standard deduction showing $26,100 rather than $27,400 for married, filing jointly, no other dependents?

The standard deduction for married filing joinly 2020 is $24,800. Add$1300 per person for over age 65 or blind. Since the difference in your numbers is $1300, just curious if one of you is does not quality for the over 65/blind deduction.

Why do they make it higher for age 65???

Ask Congress....The standard deduction has been higher for people over 65 for years.   It means less of your income is taxable.  (And....makes it harder for you to use itemized deductions....)

Tax returns are used to implement social programs, like the Child Tax Credit to help Taxpayers with children and the Earned Income tax Credit to help working Taxpayers with lower incomes. 

The increased Standard Deduction is a way to help senior Taxpayers.