It is commonly assumed that there are a handful of tax brackets in the United States. For example, when filing as Single, up to a taxable income of 9,700, one is supposedly in the 10% tax bracket. However, reading the instructions for IRS Form 1040 on page 38, it states that if ones income is below 100k, one should use the "Tax Table" provided later on pages 67 onwards. According to this table, the tax owed on a taxable income of 6,000 would be 603 (filing Single), which is more than 10% and not due to rounding. Later in this document on page 79 a tax worksheet is provided which performs the computations one would expect (but it only applies to taxable income >= 100k as per the instructions).
Why is there this disconnect between the lower and higher tax brackets? Again, the 10%, 12%, and 22% tax brackets don't seem to really exist. Instead there seems to be more going in this computation which only approximately results in these percentages. Why isn't everything treated the same way?