A lot depends on the powder and the size of the charge. .5gr in a 4.0gr charge can make a big difference. .2gr in a 45gr charge would be almost impossible to notice.
I'm reloading on my 5-0-5 RCBS scale and it alomst never weighs the same charge twice, which is the by product of my Lyman "cruncher" measuer which never throws the same charge twic
I'm confused...is it the scale or the powder measure or both? if it's the scale(and it's a good one) check it against checkweights and clean the knives. If you still are having problems, call RCBS, they'll take care of it. If it's the measure, maybe you need a baffle. Could be static, wipe the inside down with a used dryer sheet. Some powders just don't measure well. Some bulky powders like Unique will vary by weight even when volume is the same. In most cases, a .2 gr variance in Unique(+/- 1 gr) to me is acceptable.