I am abroad and my US income is less than std deduction, and I have enough tax credit for my foreign income. Why does turbox show I still have to pay US tax?

I had foreign income and pay tax with a much higher tax rate than US. I had a little US income, definitely less than std deduction. However the turbox tool shows I still have to pay some tax to US, looks to me like a double tax.
EXAMPLE US income: 20k