This is actually working as designed. Within a transaction, there exists a collection of line items. For soft credits attached to a transaction, this soft credit will also be attached within the transaction as one of the line items. So even if a query is setup to include only gifts/donations and no soft credits, when the report is exported as line items, the soft credit does get included since it is a line item and part of the transaction queried on.
Our Standard Accounting Exports are built to skip over these soft credits One of those may work as a workaround if you do not need to include Soft Credits.