The explanation is on the page you linked (emphasis added):
If you're an employee, then you pay one half of this total (probably
as a withholding on your paycheck); your employer pays the other half
for you (and then gets a deduction for their half on their corporate
tax return, since it's an expense - for them it's as if the FICA
"half" is an additional piece of salary). If you're self-employed,
then you pay the whole total yourself as Self-Employment tax, and then
get a tax deduction on half of it as an "adjustment" on your tax
But, note that YOU WILL PAY MORE as a 1099 "contractor" since you will be paying the WHOLE amount, not just half. The calculator is just baking in the deduction for the extra half that you pay.
So as an employee, you will pay $4,590 (not the total $9,180 - the employer pays the other half). As a 1099 you will pay $8,478; less than double because the other half is deductible, which reduces your taxable income.