simple tax rate question

So I'm retired and have a very simple tax calculation and end up with a taxable income of $5,550.00.  Filing status is married filing jointly.  Turbotax calculates my tax due as $558.00 NOT $555.00??  Why the extra $3.00?  Should be just a straight 10%, right?  Am I missing something?  Already filed and they were accepted but I just saw this.