Fake poached salmon machine?
After further inquiries and experimentation I have discovered when cooked salmon is left out for a long time it starts to turn dark orange at the edges, probably due to oxidation. So, possibly the browning is due to being left out a long time, not over cooking, however, it could be due to overcooking.

After I complained about steamed salmon being sold as "poached" salmon, the supermarket changed the label on it.

There are apparently steam ovens and also "combi ovens" that cook with both steam and heat. If they were using a combi oven in heat/steam mode, which a lot of places due because it cooks faster, then the salmon could be getting over cooked.