Strangely, the more "artificial" or "processed" a given food is, the more consistent it's likely to be.
Raw / natrual / organic stuff, being natural, has a higher degree of variability. So for example a stalk of broccoli can vary dramatically in how much vitamin C it has. That's not gonna stop me from eating broccoli of course.
The other thing is vitamins, supplements, sports powders are regulated as food not medicine, and I don't know if they meet the same label accuracy standards a say a bag of chips or a can of green beans.