It's an artifact.

I just looked at the waitlist for AS #38 for tomorrow. It's quite likely that the seat assignments we see on the waitlist as "Premium Upgrades" are actually the seats that had already been selected by the passenger(s) at the time of booking.

That is what happened on 2 of my flights last week. I had booked specific seats in row 7 back when I purchased the tickets. But when the waitlist opened up it showed me as having been "upgraded" into that very same seat.