I'm not sure which explanation is correct, but I tend to favor the one claiming that most juices have components of different densities and/or particulate contents; in top coil designs, the "lighter fractions" of the
juice have an easier time wicking up, whereas the denser, darker stuff remains at the bottom.
Theoretically, there should be a difference in the flavor, then, but there shouldn't be anything
wrong with the darker juice. Also, apparently, using a bottom coil clearomizer would prevent this from happening....