I'll take a stab. If you notice whoever has a (S) under a trip (I know you're asking reserve) then they'll also have a SB under their name. Maybe that has something to do with Shadow Bidding?

I found in CA 09-12 under the Delta Bidder Guide a shadow bidder: Awarded to senior shadow bidder pairings matching this bid preference were already awarded to a shadow bidder with higher seniority than you.

So, what I think it means is you were awarded something only because the person who originally got it couldn't keep it. There is SHADOW (RES) if you look under the reasons reports. But I'm not sure.