I don't know for sure, but I've heard the same explanation you posted (spraying vs vacuum/heating). If it's true, then it's easy to see why the liquid would have more caramelized/unfermentable sugar.
DME always seems to ferment more cleanly in my experience, while liquid stays sweet.
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit