The most likely reason is that you've reported in TurboTax that someone else could claim you as a dependent on their return, so your standard deduction is limited.
According to IRS Publication 17, Your Federal Income Tax,
The standard deduction for an individual who can be claimed as a dependent on another person's tax return is generally limited to the greater of:
- $1,100, or
- The individual's earned income for the year plus $350 (but not more than the regular standard deduction amount, generally $12,400).
However, if the individual is 65 or older or blind, the standard deduction may be higher.
