Weird USCIS I-797C notice of action

[Christi85 86]

I had something weird happen just the other day:

So, I became a PR in June 2011. Recently, my husband and I moved to a new place. As required, I filed form AR-11 online with USCIS to notify them of my change of address, and so did my husband being my sponsor by filling his respective form (he did his via mail, as there wasn't a way to do it online). After submitting my address change online, I got a confirmation screen, printed the form for my records and forgot about it.

About 3 weeks after that change submission, USCIS mailed me a I-797C Notice of Action that was exactly the same as what they had sent back in June '11 with my green card "Welcome to the USA" etc. etc. Together with that, they sent a Requirement to File for ROC letter (which I'm not due to do until next month at the earliest, but anyway), AND they sent both letters to my previous address, not the new one I had given them!! Luckily, my mother in law still lives in that address, so she was able to get the mail on my behalf, but I'm wondering:

a) What's the point of sending an I-797C? I don't have any cases pending right now, and besides all it said was "Welcome to the United States" etc., same as the welcome pack they had sent when I first became a PR. Would that be their way of confirming my change of address? But if so,

b) Why did they send it to my previous address since I had already submitted my change of address online?

c) I thought they do not send reminders to file for ROC. I guess they do