Why does broiled and roasted salmon give these different results?
Roasting doesn't really imply a specific temperature. The roasted salmon you've had might've been done at 450F, or perhaps a lower temperature. Either way, it doesn't require tons of browning. You don't want to overcook salmon, so you'll cook it until it's cooked through, and even at 450F, that doesn't necessarily mean a ton of browning. If it's on a bare metal pan with plenty of room for juices to get out and evaporate, it'll brown more; if it's in a crowded pan that has water in it most of the roasting time, it might brown a bit on top and none on the bottom.

Broiling, however, basically always means high temperatures. The air temperature might be 500-550F, but there's radiant heat too, and the food is close to the heat source, so it can brown easily and rapidly. On top of that, the main heat is coming from the top, so by the time it cooks through, it can easily brown on the top. Getting things crispy and browned is one of the main points of broiling.

So no, I would not expect roasting to have as much browning as broiling would. If your goal is not to brown too much, then just don't use a broiler - bake/roast at some "normal" oven temperature.

(As for "greasy and mechanical", I don't know - you could've been tasting burned oil, or maybe there was some other variable besides the broiling.)