No, it doesn't necessarily have anything to do with possible filler ingredients mixed in with the bulk ingredients. What I'm trying to say is: a scooper is not anywhere near as accurate as capsules are. When you're scooping things in mg amounts, or even single gram amounts, there can be a huge variance based on a "full scoop". For example, some scoopers have a "fill line" in them that is part way up the scooper. Filling to that line is the correct serving size, while leveling off the scooper at the top is actually more (or something significantly more) than one serving. Additionally, there can also be variances in how you scoop. Do you pack it in against the side of the container to obtain a full scoop? If so, you might be consuming more than one serving, depending on the density of the powder and the size of the scooper. Ever notice that some bulk supplies don't last quite as long as you expected them to? If so, it might be because you've been consuming more than one serving every time you scoop it (most people won't notice since bulk supplies typically take a while to use).
Now, the margin of error in the scooper (and how you scoop) generally isn't a big deal, but if you're dealing with specific nootropics or stimulants, it certainly can be.