Most dozens will show at least a .030 difference between the stiffest and weakest arrow in a group (that's a lot). I've measured some dozens where the vaiation was .065. That's incredibly huge variation, that can't possibly result in a consistant shooting dozen. If you're shooting borderline spined arrows, you are not likely to get the same point of impact from such variation, when shooting broadheads.
Straightarrow,
It would be very helpful and informative to know just what the measured difference in point of impact was that you have found with these variations in spine deflection. Also did you find a pattern in point of impact differences to spine difference (weaker spine high, low, left or right)?
Also one thing that I don't believe I have seen mentioned in this discussion that has been the biggest determining factor for me in broadhead grouping is alignment of insert to shaft. I have my own system of installing inserts that works for me and I hate to let anyone else install inserts on arrows for me to shoot. I have shot arrows with fixed broads only to have them fly all over the place, then remove the insert and install straight and have the same arrow fly perfectly. This has occured with some of the best and worst straightness tolerance and and highest and lowest priced arrows.