When it comes to Income and The Immigration Rules it's all about Taxable Income
For those who are 'employed' this is gross income on PAYE

Taxable income for Self-Employed is Revenue minus Business Expenses.

If the business accounts show that the person is being taxed on an amount less than £18600 then the application will fall for refusal.

Quite a number of people have slipped up on this.
Sometimes it may be necessary to adjust the business expenses structures and 'tax-breaks' so that taxable income is £18600 or more.

Under 'normal' conditions those Self-Employed are trying to keep taxable income to a minium.
Under Immigration Rules self-employed need to uplift taxable income.