The $12,400 standard deduction is for someone who is not a dependent. For a dependent who is single, under 65, and not blind, the standard deduction is the greater of $1,100 or your earned income for the year plus $350 (but not more than $12,400). So if your earned income is $3,905, your standard deduction is $3,905 + $350 = $4,255.
Note that it's based on your earned income, not total income. Earned income is basically income from working. However, for calculating the standard deduction for a dependent, taxable scholarship income is treated as earned income.
