A long Achilles is good for the standing vertical jump, whereas a short Achilles is good for a one-leg jump because it allows a shorter (quicker), more powerful rebound.
The thing with the highly attached Achilles (high cut calves) is that they are usually present in people that also have a bunch of other structurally advantageous factors (narrow hips, long legs etc etc). So it's not just the long Achilles in these people, the long Achilles is just a good indicator of a overall "solid" structure for jumping advantage.