The real number sits under the fees. Referral charges, fulfilment, storage and returns have to be subtracted before a product's profit is known, and advertising a line that loses money at every sale is depressingly common.
Inventory must reach the campaign. When stock is measured in one place and budget in another, a product sells out on Friday and the advertising continues all weekend, paying for clicks that reach a page nobody can buy from.
Marketplaces disagree about everything. Different report formats, currencies and definitions of a sale mean figures need normalising before any comparison, and doing that by hand each month is a lost week.
Platforms owe sellers money more often than sellers realise. Stock lost or damaged in a warehouse can be reclaimed, and an agency that checks systematically returns more than its fee for some clients.