Didn't Rand initially Skim as a rising stair? Maybe Skimming is upward motion, but no one knows because they don't have any visual reference points. Did we ever get a PoV from a passenger? IIRC, we only have the channeler who made the gateway "sensing" the motion. Maybe the people who saw the Aiel guy fall only thought so because they were rising and had no sense of motion, so they assumed he dropped.