IF you are self-employed, They may be including Self-employment tax (SS and Medicare)  that is just at  15.3% of your SE profit. 

That SE tax is applied before your personal deductions are used to reduce your AGI to the "taxable income" level.

Thus, a married couple  could have all Self-employment income of 38,500

then Roughly:

~15% of that is $5775 SE tax

...then 38,500 - 24,000 std Ded for MFJ = 14,500

...tax on that is ~$1450

Total tax :  5775 + 1450 = 7225

= 50% of taxable income

= 19% of AGI

(50% + 19%) / 2 =34.5% blended?   a WHOLE LOTTA Handwaving there 

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You gotta decide what number you want to focus on...not sure the "blended" number is meaningful, but then I'm just guessing as to how they calculated it. A simple   % of AGI would be more meaningful to me.