If you had known you'd be concerned about accuracy, you could have done a rollout measurement on the old tires before getting the new ones. If you still have the old ones, you could still do it -- remove one wheel from the truck, deflate the tire, and do the rollout on that compared to the old tire, I suspect that would be accurate enough.
No, I don't have the tires anymore, and the only reason I'm concerned with accuracy is for my gaslog. I suspect that because the tread is so deep on these tires that my speedometer and odometer readings are off. I guess I'll have to check it against my GPS.
Tread depth, and just manufacturing differences...even from model to model sometimes tires with the same size markings differ slightly in size, and from manufacturer to manufacturer they definitely differ.
Did you check your old tires against the GPS? If not, it may never be possible to get an accurate comparison.