I made about $4500 last year as self-employed. This was my only income. My standard deduction is $6350. Why do I still owe $522 when I'm within the "tax-free income".

I only made a total income of $4504 in 2017. All of that $4504 was earned being self-employed. I am filing using a 1099-MISC. My standard deduction is $6350. If I understand it correctly, a standard deduction is supposed to be the amount of "tax-free income" that you can claim. So why is it that even though I made less that my standard deduction, I still have to pay anything? The standard deduction should apply to all of my income should it not?