Why doesn't the total tax before credit reported by turbotax not match the value from the IRS tax table, when using the taxable income calculated by turbotax?
There is more than one tax table. Do you have, for example, any capital gains or qualified dividends?
If your total tax before credits is smaller than expected, then your tax may be calculated using the Qualified Dividends and Capital Gains Tax Worksheet instead of the tax tables.
If your return included either qualified dividends or capital gains, then some of your income is being taxed at a lower tax rate than the rest of your ordinary income. If this is the case you will find the Qual Div/Cap Gn worksheet included with your return. On that worksheet, the tax from the tax tables will be shown on line 24 and the tax based on the worksheet will be shown on line 23. The smaller of these two values will then be reported on Form 1040 line 16.