Why doesn't my Tax, line 11A, not correspond to the Taxable Income listed in Line 10, based on the tax brackets?

[TerryA]

If your taxable income is less than $100K be sure you are looking at the 2018 tax table. If more than that the tax rate schedules are used. Otherwise...

If you have capital gains and/or qualified dividends then the tax could come from one of those worksheets due to reduced tax rates for those items.

Or if you are a certain child who has unearned income (Form 8615).
Or you are a parent reporting a child's interest and dividends (Form 8814).
Or if you are a farmer/fisherman and you are income averaging.
Or it could come from the Foreign Earned Income Tax Worksheet.
Or you have a tax on lump-sum distributions (Form 4972).

plus some other obscure tax sources..

If you can get a look at the 1040 Worksheet at the top of page 3 is the Tax Smart Worksheet. That will indicate the source(s) of the tax.