Not sure there's that much variation within each brand to have the Volkls be perceived as the softest or the stiffest. After all, the shorter junior models are AFAIK made by Fischer or Elan; both companies are known for their quality control. Moreover, the kids themselves seem to agree on the rankings I suggested, not just the coaches.
Rather, I'd guess it's subjective perception on the part of us adults projecting (the Volkl adult models with UVO and the Marker World Cup plate are fairly stiff, for instance). And agree that since Nordica and Blizzard are the same ski, same factory, different topsheet and trim, as are Dynastar and Rossi, any "differences" are us feeling what we expect or want to feel, not the actual physics of the skis.
Yep, short and long will have different skier mechanics. If WC racers had their way, they'd be on 150's. FIS mandates a minimum SL length for its own reasons, presumedly having to do with course design, manufacturer pressures, and oh yeah, safety.