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SodaPop,
The quick and dirty answer would be that some brands of brass can tend to run thicker than others especially in the head/web area.
A stark example can be with Military brass which in most cases tends to be thicker in it's walls. Thus, requiring charges to be reduced, due to less internal volume, with max charges reached sooner.
From brand to brand, the difference can be substantial at times.
That's why when changing brass brands or lots, it's best to reduce your charge and work up again if you are into a max loading.
Some brass also tends to be "softer" and others "harder". However, affects performance to a lesser extent than wall and web thicknesses.
There are other variables such as primer pocket depth, rim thicknesses, annealing quality in the neck area that can also have an effect on performance of the brass/loads.
Regards
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