You also could have air motions, electric fields and so on passing through the trajectories of the coins. Unless you account for everything on atomic level, you

You also could have air motions, electric fields and so on passing through the trajectories of the coins. Unless you account for everything on atomic level, you have all that uncertainty as random noise in you measured outcome. So indeed it is a random variable. Biased, yes, but random. Actually, even if you can have data at atomic level you still have chaos theory and butterfly effect preventing you from making prediction. Then even some small fluctuation born by the Plank's "uncertainty principle" can grow up in time to big enough error.