Ultimately Sales/Revenue/Income = Amounts Billed/Paid by Customers, agree?

The difference between reports is usually a matter of timing and how you run reports.

For example,

P&L run on a cash-basis will only include revenue where you have already received cash.

Accrual-basis would include revenue that you have billed to customers who have not paid yet.

Sales by Customer Summary should match one of those P&L's, so you can see the differences.