Hey bud...okay here we go!
Please understand that this is all total theory, and nothing is really proven. I am not a scientist, but i am an observant farmer.
Yes i totally know what you are saying about the shifting sun affecting the amount of sun you get in a spot like that. Try to find the spot with the most light on the average, or move pots to maximize exposure.
Its weird.....its doesnt make sense why they do that for me, but it does indeed seem to be a pattern. The nightly cold is something i deal with all the time here most seasons(45th cannada), and the partial sun garden location is more sheltered form cold winds ect than the full sun garden plants that always seem to finish close to quoted breeder finish times. The more i have grown autos outdoors, the more it makes me think thats its the limited light that are slowing the process.
Autos outdoor have the potential to get quite large if the climate is good. Even with your estimated 6 hours of direct sun they have the potential to get quite large. I've had a few 40"+ autos in the partial sun location(which gets the same sun as your spot 6 hours at best here), but generally they get to around 25-30" in height. Buds are almost always fluffy(lack of density), but very solid potency. Versus their full potential indoors though on a 20-24 light schedule though? Outdoors in limited light are only a shadow of an autos true potential. In full sunlight(15 hours), even with some adversity to deal with(cold nights, high wind, high RH%) autos are capable of exceptional things.
But limited sun versus indoor full light in a controlled climate is no comparison. There is guys here that consistently grow 8oz+ autos in fine indoor setups.