Am I the only one Delta seems to want to foist unsolicited Skymiles upon?

Yesterday I traveled on a DL ticket purchased on DL.com.

Outbound on NW, inbound on DL, all on the same day.

For the sake of saving a few keystrokes, I plugged in my DL account number on DL.com before making the purchase, but later added my preferred account number (CO) on the PNR over the phone.

When I checked in with NW, all my boarding passes (NW and DL) printed out with the correct program number on them. However, when I got my DL BPs reprinted, they spat out with the original DL account number on them. When I asked the agent if I could get them corrected, she told me not to worry - the CO number was in there (I know, I know - )... indeed, she showed me the PNR and there was my CO account number, plain as day, with bold ***ELITE PLUS*** notation next to it. The DL number was further down in the PNR. I expressed my skepticism and to her credit she punched awway at the keyboard trying to make things right, but two reprints of the BP could not shake the DL number.

Today, I see the DL segments have posted to my little-used DL account as I kind of expected... but so have the NW segments.

Why would the NW segments post to DL when the BPs clearly say CO?

Is this a DL.com problem? This is the first ticket I ever bought on DL.com and I have never had this problem before.

Thanks for any info.