My theory on that is that in a diffuse light environment, such as cloudy a lot of the time as in Seattle (from what I have heard, don't know if it's really true), it doesn't matter as much which direction or which wall it's planted against. Just matters how much clear sky is overhead. If there are no clouds in the sky, then most of the light is going to be coming from directly from where the sun is. But the clouds diffuse and spread out the sunlight so that light can come from the north, etc, in other words from all around, anywhere you can see the sky. You can see grass growing very happily right next to a wall that is directly to its south, meaning that it never ever gets direct sunlight. The only way it is getting light is from indirect light reflected/refracted by the clouds. Obviously peak brightness is not as much as if it were in direct sun.