I wonder too...I've noticed that 11/32 bohning classics are tighter then 5/16ths on the same string. COnsider a casual observation lol....but it sure seems to be true imho. Which in theory would make the 11/32 arrows act stiffer.
What tapering I have done it has weakened the static spine a few pounds depending on how long of a taper I've gone.
Another thing that might come into play is physical arrow weight. I havent played (YET) with the sure woods, have some on the way, but if they are quite a bit heavier they are going to act stiffer, then a lighter arrow of the same spine.
Add a lighter shaft, looser nock, and slightly reducing the spine by tapering...the three together could easily add up to a weaker shaft then expected.
Where's Bob when you need him ?!?!