Because a payout is not your sales. It is your sales, minus what the marketplace took, minus what you refunded, for a period that does not line up with your sales period.
Those are four separate differences, and they are all normal. Here is each one, and where it goes.
1. Fees came out first
The marketplace deducts its cut before it sends you anything: final value fees, payment processing, listing and store subscription fees, promoted-listing costs, sometimes an international fee.
In your books these are expenses, not a smaller sale. A $100 sale with $13 of fees is $100 of revenue and $13 of expense — not $87 of revenue. It matters because the two produce different gross margins, and because you cannot deduct an expense you never recorded.
2. Refunds came out too
A refund issued this week reduces this week's payout, even when the original sale was last month.
Refunds are recorded as a contra revenue account — a deduction from revenue rather than an expense. That is what makes the difference between "gross sales" and "net revenue" on your Income Statement, and it is why those two lines are shown separately instead of as one netted figure.
3. The timing does not line up
Marketplaces pay on their own schedule. A sale on the 30th is usually in the next payout, not this one. So even with no fees and no refunds, a payout for a period will rarely equal sales for the same period — the two windows contain different orders.
This is the difference that most often looks like a bug and almost never is.
4. Money is still being held
At any moment the marketplace is holding funds that are yours but not yet released — completed sales awaiting their payout date, and sometimes a reserve.
That is not lost money and it is not revenue you have not earned. It is a receivable: an asset, sitting on your Balance Sheet as eBay Receivable or the equivalent for your channel. When the payout arrives, the receivable goes down and your bank goes up. No revenue is recorded at that moment, because the revenue was recorded when you made the sale.
How to check it yourself
Open the register for your marketplace receivable account. It reads like a bank statement: sales increase it, fees and refunds decrease it, payouts clear it. If the closing balance matches what the marketplace says it is holding, your books agree with reality — regardless of how different the payout and sales figures looked.
That is the check worth doing, and it is the one that actually proves something.