Someone on another forum did a test using different scoops and they did get slightly different readings but generally only differing by 1 drop or sometimes 2 if the transition was on the edge between drops. The reason is mostly that with less dye the transition from light pink to clear is a little more subtle. There is an equilibrium between the chlorine in the water and that which has reacted with the dye (though the reaction is mostly towards reacting with dye) so with more dye the intensity of color at any given chlorine level (even a low level) is higher, though not proportionately. So it is potentially more likely for someone to miss the transition because it is so faint (i.e. they think something is clear when it is really just very faint pink). In other words, the issue isn't with the chemistry, but with your eyes and how clearly you can see more subtle transitions in color.

In practice, I use just one heaping scoop in a 25 ml sample and don't have a problem seeing the transition, but that's already a 0.2 ppm resolution so not a big deal. For those using a 10 ml sample, the transition is much stronger anyway since one drop represents 0.5 ppm so again, not a big deal.