You’re also assuming that there is absolutely zero air inside the vial. Which as we can often see-- there is always some-- and tiny amounts are introduced every time you use it… it doesn’t take much. An air bubble the size you’d find in an insulin pen could render your vehicles brakes totally worthless, and that’s a much larger system— analogy just to show how quickly a tiny bit of air would disrupt the perfect system you describe…
And of course the plunger itself is made from a soft material with some small amount of compressibility/ expandability… and the plastic vial itself would only have to expand or contract a tiny tiny amount in order to allow one tiny droplet to squeeze out the end…
I’d suggest there’s plenty of slop in he system to allow tiny amounts to be vacuumed out