I think that it is possible that your necks are just a touch longer than the chamber. This would account for the hard bolt close, and puts a slight crimp on the bolt. This would drive up pressure, which drives up velocity. Also, it is possible for the case to shrink in length if it is tight on headspace dimension. As the case expands it 'bulges' slightly, and reduces the overall length.
This is pretty easy to check by taking the outside dimension of a case right at the tip of the neck, and then chamber it, and then take the measurement again. Could be a brass length issue, or potentially even the neck portion of the chamber is cut a little short. I would say that there is a better chance that it is the former, and not the latter.