But it barely matters, it's not like I'd inject the whole cup into my veins and then see what happens.

If the caffeine was too high (within reasonable variations), you'd notice and slow down consumption before it mattered. If it's too low, you can just drink more. You can do it by feel, which is I think what just about everyone does.

Some of it might be placebo, but if so I'm not sure it changes anything really, since the goal is just to feel a certain way anyway.