Discrepancies between the Sales by Customer Report and Income Statement Report caused by Discount Items.
One of the possible causes of discrepancies between the standard Sales by Customer Report & the standard Income Statement - Sales portion are the Discount Items on the Invoice transaction.
The Total Revenue field on the Sales by Customer report is net of the discounts applied on the line items of the Invoice.
Meanwhile, the Sales portion of the Income Statement may or may not be net of the discounts applied on the invoice. This is depending on the Account setup of the Discount Items (Discount Item > Accounting tab > Account) and the respective Account Type (Account > Type).
Hence, if the Account Type of the Discount Item is different from the Sales Account, it would not net on the Income Statement - Sales section, but appear on other sections of the Income Statement (e.g. Other Income / Expense).