I'll add that you can see visible differences in quality of tablet production.
Sun, part of a multibillion dollar pharma group, for instance, produces very hard, slightly shiny, white tabs. Essentially the same properties you'd see in any white tablets you'd get in a US pharmacy.They break apart cleanly on the split line, and are typically smaller than the same dose from competitors. This tells me they have quality presses, and likely formulate excipient recipes to minimize the amount of other ingredients that have to included with the active ingredient.
HAB pharma, a good company but much smaller; makes decent modafinil, but you can clearly see the difference side by side. Their tablets are dull ("chalky"), and have a tendency to crumble, sometimes just while being pushed out of the blister pack. In general their tablets are larger at the same dose. Again, they're fine in my opinion, but Sun is a step above.
As you go down the line to even smaller companies like Winlife, their tablets get really crumbly and clearly aren't made as well.
Finally bottom of the barrel companies like "Orange City", that are little more than legal drug dealers basically (they focus on pain meds you'd expect addicts to use), make really poor quality pills. They seem to use the same excipient for everything, that has a weird garlic taste and smell.
Now none of these companies are putting out bunk, but there are other factors that affect potency, not just the amount of active ingredient, including excipients used, size, and dissolubility of the tablet.
Finally, some vendors ship old stock that appears to have been kept in hot, humid conditions, so the pills are discolored on arrival, which undoubtedly spoils them to some extent.