I have noticed this over the years, last year my mom made platinum (with my help) by 4 miles. The NWA itineraries are not correct. I've been so sick of hearing flight attendants on the MSP-SEA flights say that "Today WorldPerks members and members of our airline partners will earn 1,395 miles" I just want to jump up and be like no, actually you'll earn 1,399 miles!

The correct mileages are posted in the NW .PDF timetable that is available on nwa.com:

http://www.nwa.com/travel/timetable/