That's because your tax due is a percent of your adjusted gross income. Your supplies are lowering your business income which lowers your AGI dollar for dollar but it does not lower your tax due dollar for dollar.
If you look at line 15 which is your taxable income, then look at line 16 which is your tax due for a better visual understanding of how it works.
You can preview your 1040 by clicking Tools>>Tax Tools>>View Tax Summary>>Preview My 1040
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