Thank you Dan Lehman,
The "short" (versus "long") adjective describes the set and hand-tightened knot, what we have in our hands just after we finish it. Now, as the knot is put under stress, it is deformed, and the difference diminishes. However, on two Double Harness bends with parallel ends, one "short" and one "long", tied with identical ropes and loaded with identical weights, and put the one besides the other, we will still be able to observe the length difference. That is why I have used those adjectives, in absence of anything better...
In the "short" version of the Double Harness bend, each tail is pushed onto, and squeezed by, the (one) leg of the other rope s collar (the leg of the Standing part of this rope), and not onto the other tail. This might spell a difference in the knot security, but I really can not tell without experiments. To my eyes, the tails are more encircled and better fit into the knot s nub, in the case of the "short" version, but that might well be only a naive impression.
P.S. (2013-11-27)
A similar "Short" and "Long" dressing of the point-symmetric Double Harness bend is shown at the attached pictures. ( One can see that the difference in length is even smaller than in the case of the Double harness bend with parallel ends ). At the "Short" dressing, where the tails of the two links overlap each other, the curvilinear paths followed by the lines are smoother.