Tax line distributions linked to PO

When processing an invoice that is matched to a PO, we set up the tax as a separate line item and don't match it to the PO.  We noticed that after creating the accounting for the invoice that when we look at the tax line distributions it is showing the PO Number and PO Line Number.  Why is the tax being matched to the PO?  Because of this our Purchasing Activity report is reducing the PO amount by the tax amount.  Are there setups that we don't have set properly?