Take a closer look at Line 2 of the worksheet.  If earned income is <=$750 then the standard deduction is $1,150.  But with the earned income it makes unearned income over $400 subject to tax.  Depending on the nature of the income you may get QD/LTCG treatment but it sounds like that's not the case here (or the system wouldn't have given you 10% x $600 tax.)