Either they just forgot, or the NPD is no longer following the NRF's calendar. Considering they've done the latter since the 90s, I don't see any reason for them to stop. I'm inclined to think Welfare and Donny are simply in error.
EDIT: More proof the NPD follows the NRF calendar:
You was right Shadow, this is a 5 weeks month, not 4. My apologies.
I'll never doubt about you again :p