It may just be an alternate route, or a route for a road, who knows. The border between Canadian Shield (ie: rock, eskers and other road building material) and the Hudson Bay Delta (ie: muskeg and blackflies) lies somewhere in the middle. The westerly route may just be sticking to more solid ground. Just an opinion based on what I've seen from the air. Its hard to tell exactly with that map where exactly the geography changes. Does it cost more to put a road through swamp, with less bridges, or over rock with more bridges and turns to avoid lakes?