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But let's say you get the same brass, bullets, powder, and primers as a factory round, and you load rounds to the identical specs of the factory round. You've eliminated all the variables. You should be able to expect the rounds to perform identically to factory ammo, without the usual routine of working up to that load.
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Each lot of gunpowder is different. Powder suppliers, such as Accurate, IMR, blend lots of fast and slow powder so the last lot of, lets say AA2495, is close to the last lot you bought. Accurate Arms told me that they blend their lots to 5% but that the industry standard is 10%. So different lots of powder are going to have different pressure curves.
We get blended powders. Ammunition manufacturer's use the unblended stuff. It is cheaper and they have the equipment to measure the pressure curve.
I have done some searching on the pressure variances due to primers. There is data out there and it is surprising to see numbers like 7000 psia and 4000 psia max and min's for primers. Given that primer cake is mixed by hand, and the ingredients in vary, because everything varies, the next batch of primers you use will be different from the last.
Of course brass varies in weight.
There are a lot of varibles in ammunition.