Why is Turbotax incorrectly calculating my self employment tax rate using 14.13% instead of the IRS 15.3%?

Because the actual formula is....

You pay 15.3% SE tax on 92.35% of your Net Profit.  You have to first multiply the profit by 92.35%.   I think it's so you don't pay tax on the tax.  

Look at your Schedule SE line 4a.  Here is a blank one Schedule SE

https://www.irs.gov/pub/irs-pdf/f1040sse.pdf

Look at Schedule SE to see how it was calculated.   7.65% of net income does not get included. 

@DawnC   Is that for the employee or employer part?  Probably the EE part since you are getting the separate deduction for the ER portion.  

You are paying 15.3% for……

SS for employer 6.2% (up to 160,200 wages & profit)

SS for employee 6.2% (up to 160,200 wages & profit)

Medicare for employer 1.45% (on all wages & profit, no max)

Medicare for employee 1.45% (on all wages & profit, no max)