25% is the base rate and is applied directly to the wages of non-owner employees. However, the self-employed individual's employer contribution itself reduces the amount which gets multiplied by 25%, so the base rate must be adjusted to account for that. The adjusted rate for a 25% base rate is 20%, so 20% appears on the worksheet as the maximum. See the rate adjustment calculation in Chapter 5 of IRS Pub 560:
Adjusted Rate = Base Rate / (1 + Base Rate)
