Historically, there has been a difference in the Soft Rounding calculation between Sales Orders and Purchase Orders when using an Item Group (Purchasing Taxable) for Multiple Tax Rate configuration. In the Sales Order module, when Sales Update (R42800) creates Accounts Receivable records (F03B11/F0911), a Soft Rounding for Tax Calculation is done grouped by Tax Area and Item Group. In the Purchase Order module, when Match Voucher to Open Receipts (P4314) creates Accounts Payable records (F0411/F0911) a Soft Rounding for Tax Calculation is done grouped by Tax Area only. The Item Group (Purchasing Taxable) was ignored.
An enhancement is now available in releases 9.1 and 9.2 that allows users to choose to enable Voucher Match Tax Rounding by company for Accounts Payable.
For more information see Enhancement Available in Match Voucher Match to Open Receipts (P4314) to Allow Soft Rounding for Tax Calculation Grouped by Tax Area and Item Group (Doc ID 2475356.1).