1. In almost all productions skis, the amount a rocker profile adds to length by having a curve to flatten is just not enough to make any difference. Maybe 1-2 CM of length? Not to mention that the rocker profile takes away the tip pressure found on a cambered ski, which is going to matter a lot more for hard-snow performance than the amount of edge in contact with the snow (comparing a rockered ski to the same length full camber ski).
2. Manufacturers commonly lie about ski lengths to balance what they feel the best design length of their product is vs. what the general ski buyer thinks they want- one example is K2's factory line of a few years ago having "189 cm" lengths while really being almost 200 cm in reality- its really hard to have an apples to apples comparison. The ski gets stamped with 189 because K2 wanted people looking for a ski in that range to consider their ski. The skis were longer because K2 apparently felt the skis would do better in the market at that true length.
3. Consider that many skis have a rocker profile aggressive enough that the tips are just not going to be in play on hard snow in a turn.
4. Consider that with a 5 point ski (Rossignol S7 and all of the other skis made with this shape), the design of the ski prevents the tips and tails from engaging. The tapered tip and tail design (aka reverse sidecut tip and tails) means they cannot contact the snow in any effective way on hard snow. Maybe they can contact if you rip a deep enough trench in the hardpack for them to touch. The running length of those skis (measured at where the sidecut portion of the ski begins and ends) is a much better indicator of how much edge there is to play with on hard snow.
In short, a rockered ski in hard snow is just not going to feel longer, in the turn or otherwise, than a full cambered ski, everything else the same.