Why is my standard deduction $5,774 instead of $12,400 for a single filer?

Are you being claimed as someone else's dependent?

 The standard deduction amount for an individual who may be claimed as a dependent by another taxpayer cannot exceed the greater of $1,100 or the sum of $350 and the individual’s earned income (not to exceed the regular standard deduction amount).

Are you a dependent?

If you can be claimed as a dependent by another taxpayer, your standard deduction for 2020 is limited to the greater of: (1) $1,100, or (2) your earned income plus $350 (but the total can't be more than the basic standard deduction for your filing status).

If your standard deduction was $5,774, it sounds like it was $5,424 of W-2 income plus $350.

Related Resource:

• Topic No. 551 Standard Deduction