We were told at the DL RWT/DO Thank You (see DL forum) that this may be a combination of the NW miles and DL miles earned this year. It appears you are one of a number of folks that, when combined, now have reached the next level of Platinum because of that, effective 10/1 (or here, 9/30). So you must have earned 35K WP EQM on NW and 40K SM MQM or so on DL this year?

Be glad they are doing it now, and benefit from it the rest of the year.

Now, to change your res from Gold to Plat, have a res agent remove, end, and add back your number to your res, and it *should* change. It may not in the res itself, but the DL mileage at least will show Plat Bonus, from my similar experience.
DL IT staff have been notified this past week that at LEAST the flyer's status level should be checked before / during checkin and upgrade times so that BOARDING PASSES should show the proper new status, as it may affect one's entry into a SkyClub / SkyTeam lounge as an international passenger, etc.
-- I think Upgrade times are based on your profile, and if there is a res tied to it, rather than the (old) status reflected in the res, as I got my upgrade notice 5 days out.