Okay, I'm not exactly disagreeing, but!- what makes you think the brass expands from the rear forward? The rear of the brass contains the web, the thicker area. Plus, you seem to be saying the powder must burn from front to rear sort of throwing the pressure to the front, but in larger cases using small amounts of powder the powder may be laying in a line across the bottom length of the case, or it may be fully forward or rearward. I don't think there's any certain rule to how the case expands unless it's a case load of powder and even then I'm not it'll always expand exactly the same. I would think virgin brass expands a little differently than brass on it's 10th loading or freshly annealed brass.
In the end, it's just another variable we can't always control, isn't it?.