Thanks and regards!
The tax tables are created from the tax rates. Its not as simple as multiple taxable income by the rate for your tax bracket, due to the way tax brackets work. The rate for each tax bracket only applies for certain income ranges. For example, the first $8,375 are taxed at 10%, $8,375-$34k at 15%, $34k-$82,400 at 25%, etc..
So just because you are in the 25% tax bracket doesn't mean that you are paying 25% on your entire income. If you have 82,400 in taxable income, and assuming the singe rate, your tax is 16,788 for an effective tax rate of 20% of taxable income. Your effective tax rate is actually less than that if you base the calculation on your adjusted gross income.
Your effective tax rate is simply your total taxes divided by your total income. The tax rate and the tax table are based on your taxable income, which is your gross income minus any deductions and credits.