If you were employed and making $100,000 you would have to pay 7.65%, or $7,650, and the employer would also have to contribute $7,650. That means that the total amount of money your employer would be paying would be $107,650, while you would be receiving only $92,350 after the tax is withheld ($100,000 minus $7,650). The $15,300 total tax is 15.3% of the $100,000, but it's only 14.2% of the $107,650.
The reason it comes out to 14.2%, lower than the 15.3% you would get if you were to naively add the 7.65% for individual contribution to the 7.65% for employer contribution, is that there's several different ways of calculating taxes at play. When you are charged $7,650, that's calculated as a percentage of your total pay of $100,000. But of that $100,000, $7,650 is taxes. So you're in a way, you're being taxed on your taxes. The $7,650 that your employer contributes, however, is based on only the $100,000 that they're paying you. They aren't charged taxes on the full $107,650 that they're actually paying. So while the two contributions are both expressed as being 7.65%, in reality there's a significant difference between them. Of the money your employer is paying to employ you, only 7.1% of it is going to taxes, while 7.65% of what you receive is going to taxes.
So if we were to calculate what the tax on $100,000 of self-employment income would be equivalent to the tax on employment income, one way to do that is to figure out what take-home pay would be equivalent to having a total employer payment of $100,000. That is, if the employer's contribution of 7.65% of the take home pay, plus the take home pay, is $100,000, then what is the take-home pay? This can be solved with algebra as
take_home + take_home*.0765 = $100,000
1*take_home + take_home*.0765 = $100,000
(1+.0765)*take_home = $100,000
take_home = $100,000/1.0765 = $92,893.64
However, the people designing the tax code apparently decided this was too complicated. So instead of asking "what take home would we have such that 7.65% of the take home pay plus the take home pay is $100,000?", they ask "what would take home be after we take off 7.65% of the total?" Note that in the first version, we're subtracting off 7.65% of the take-home pay. That is, I calculated that the take-home pay is $92,893.64, and 7.65% of that is $7,106.36, and when we add that back to the $92,893.64, we get $100,000. But in the second version, they're taking away 7.65% of not the take-home pay, but the $100,000. So instead of taking away 7.65% of the $92,893.64, they're taking away 7.65% of the $100,000. This means that your taxable income is lower, which reduces your tax rate even further. So instead of the 14.2% rate I calculated earlier, your tax rate is only 14.1%.
So, I think the take-away here is that you shouldn't ask question about tax calculations unless you're prepared for a long, complicated math-filled answer.