Well, I hate to be wrong, but I was wrong... Yeah, I can't still work the geometry perfectly, but in some way my mind already agrees that the way I drew it makes no sense. I did some searching, of course, of counter-examples, as a stubborn arze that I am, and couldn't find any - here's the pic that settled it for me:
shadows_study_torrevieja.png
Screen grab from google.earth, a church in a mediterranean village of Spain, called Torrevieja. The roof is clearly more inclined than the sun rays, and yet, all flat surfaces have equal lighting (I did pan around the city, in despair, but it's always like this!). I was indeed drawing the shadow of the roof peak, but the thing is, as I see it now, with your much appreciated help, in a flat inclined surface, each "line" is the peak for the line immediately below, hence all points are under the same "shadow" from the next point above.

In a way, again, I'm a little glad that I redoing my shadows.
And in the meanwhile, many thanks for existing and being the good people you are, folks. This forum is awesome in the way it allows (and drives!) us to learn stuff.