I called back and asked another agent to check the notes, and it turns out that Flying for Fun was right with the first guess: the notes say equipment swaps. The matching seat numbers, just coincidence.

I saw in another of the seemingly 500 threads I've read here in the past couple weeks that some flyers recommend not selecting a seat with a Saver Fare even when they're open for selection. Odds of getting something better at check-in or gate assignment may be good. My experience last weekend worked out that way. When the next sale comes around and I book cheap MRs, I'll probably do that (forego seat selection) on half the segments just to see if something like that happens again, row 11 for row 32 prices.

Thanks for the suggestion.