Employer has $125,000 to give to employee.  Gives wages of $100,000, plus 25% for SEP ($25,000).

Self Employed person has Net Earnings of $125,000.  To keep things equal, SEP is still $25,000, which is 20% of Net Earnings.  That is why Self Employed people use 20%.

As was pointed out, after factoring the 1/2 SE tax deduction, it actually works out to something like 18.57% of SE profit.