The case weight variation effects case capacity and therefore burning rate of powder.
The .02 gr. variation in powder charge is considerably better than factory standard, where 1/2 gr. variations can occur.
Your case variation example would be a 3% (within factory standard) variation and sorting your brass can easily get you into 1.5 gr. tolerence + or - from avg. I'm using an '06 case for an example.
In an '06 case, it would take 1/2 gr. variations in powder charge, or more before you could, maybe, see it on paper--- if you were a good shooter-- using an accurate hunting rifle. That's 1/2 gr. variations from average,which means there could be a full gr. difference in the extreme.
In a .223, the .2 gr. pwd. charge variation wouldn't be noticed, but the 6 gr. case variation would be too extreme, if even possible. Better to go % of average with 3% being ok, and within 1% a little too strict and unnecessary--- unless you're a benchrest kind of guy.
Handloaders don't have to put up with those variations. You can sort/weigh your brass to better than facory standard and load powder charges dead on the money if you weigh your charges.
I assume you're talking about rifle loading, but even if loading for pistol with those kind of variations, you wouldn't have match ammo, but you'd be ok for serious practice.