Probably different case volumes are having an effect.
That makes sense if there is a noticeable difference in volume. I am one of those people who will adjust by 1/10th grain of powder and keep a chart of the most accurate load for a particular rifle ( although I admit for most applications I try to find a common denominator because of having so many chambered the same ). I haven't experienced, or maybe haven't noticed, too much of a difference between brass manufacturers. I try to stick with the same manufacturer but even when shooting mixed haven't noticed a big difference in accuracy between identical loads. Maybe this means I should pay more attention
