jinabe, your proposed method of calculation would be absolutely correct if you'd used a $75 voucher toward the original ticket at the published fare. But you used a coupon to reduce the original fare, then paid the reduced fare. If you look at your original receipt, it will not show the $75 coupon anywhere, but will merely show the reduced fare as a "promo fare" (indicating that some sort of special discount was used to reduce it below the regular published fare).

It definitely sucks to lose the value of a coupon like this, but unfortunately that's how NW does it. There have been reports on this board before of folks using PD coupons, then changing tickets and being unable to recoup the value of the coupons. Sorry we can't be of more help.