Joe
The 236 bore probably has an initial higher pressure spike. This is not much different than seeing 100 fps difference in two different styles of identicle weight bullets over the same powder charge.
As an example, if you are getting 3400 from a Barts Ultra, or Bruno Boat Tail, (examples of 00 bullets), and switch to a Fowler Style Bullet, (7.5 o-give, relative straight shank), you will see 3500.
I have always considered .237 4- groove Kriegers "slow barrels". They tend to act a little loose, especially when combined with one of the 00 bullets. That is why they tend to respond so well to upper end loads........jackie