The third level has steel beams supporting the corrugated decking the second level has bar joists. I suspect the third level is a concourse level with anticipated heavy live loads (Standing room only type if loads). I need to check the code but i would suspect those loads to be well above 100 pounds a square foot. I suspect the second level is more of a support type of occupancy with lower live loads. Bar joists are notorious for being bouncy because they are light weight. I suspect the designer anticipated large live loads on the third level that could be moving in unison and could cause a harmonic oscillation. Something that would cause discomfort for the occupants and could cause motion sickness in severe cases.