I actually thought this was a "bug" with the renderer, feel silly now realising it's a feature, a great one at that, now it's been pointed out! very cool. Anyone wondering if worth some play time, it definately is!
It definitely feels like a bug to me, and would be the first thing I'd turn off. I mean, I get that from that one angle it looks flat -- but the sun moving with my camera would be way, way worse for me.
I wonder if you rotate the camera more often than I do. I can go an entire play session of Anno without rotating the camera once, since it's a top down city builder.