I wouldn't say its standard, but it does happen regularly enough. As a self-employed person your case is not straight forward so you're more likely to need to be interviewed.
They will go through the application and your income and question you about other possible income that you may have. They may delve a little into your business expenses to make sure that they relate to the business rather than personal expenditure.
I take it that you have the necessary 19 hours employment on top of whatever self-employment that you have, as self employed hours don't count for FIS. Income from self employment however does count in the calculations, and I imagine that's what they're exploring.