I think it goes pro rata. Thinking about days they dont work due to their period, sickness,holidays circa 90 days in a year off work and if they are off then there is no money coming in so 365 - 90=275 working days. If declared 20k it means the average earning is circa 73 per day so more or less 438 weekly salary. Those who earn more have probably higher expenses therefore writing it off the profit. Maybe it’s just easier for them not to declare amount of money that they would have to write off anyway like £500-800 for working flats a month, hotel fees, petrol, car ,cosmetics ,lingeries, advertising, god knows what more.
It’s not a gold mine but def stressless and easy job for them without having a boss and monthly meetings.