Because these simulations contain such a very, very small fraction of the actual number of solvent molecules present in actual solutions, their random movement across the membrane occasionally results in temporary volume fluctuations. Occasionally, a few particles, but a relatively large fraction of those present, diffuse out of one compartment and into the other without any returning the opposite direction, producing relatively large volume fluctuations. In much the same fashion, flipping coins may produce short runs of heads (or tails), but over billions of tosses, the ratio of heads to tails rarely deviates from 1:1. Thus, the volume discrepancies here are artifacts of the small number sampling technique we're using to illustrate solvent behavior.