Re: annealing velocity change???
Several factors could contribute, like temperature, a different lot # of primers, powder, etc. One thing I noticed was a drop in velocity going from new brass to once fired in my 280 Remington with Norma brass. I went from 57.5 grains of powder for virgin brass to 57.7 in once fired just to maintain the same velocity.
I do not know honestly but would think annealing would be closer to the bottom of causes for what you are seeing.