If you're exporting our Orders report and our Item Details report, you'll notice that each report has a "Discount" column. You may also notice that the total of these respective columns may not always match.
The ItemSelectionDetails report reports each individual item that was ordered and the discount column is the amount by which that individual item was discounted. This mean it only considers item-level discounts, not check-level discounts.
The OrderDetails report reports each full check/order and the amount by which it was discounted - which includes check-level discounts as well, unlike the other report.
As a result, the OrderDetails discount total is the true total number to refer to as it will include both check and item level discounts.