It's official. The back left side is about 1in lower than the back right.
Hmm now that you think of I think i may know what it was.. The rear sway bar seems the most likely because it was lower in the back than the front, does that seem like a valid reason? It did seem that the rear sway bar wasn't really on there very well..The stock springs are rather consistent, if not perfect.
I'll bet that on installation of the aftermarket springs, you re-tightened something in a position that was more accurate than off the assembly line.
It wasn't the actual springs themselves.
The springs are produced by the thousands like perfect clones.
Not much likelihood of variance there.
It was almost positively an assembly error.