The two rails here are configured differently along a few dimensions. The top one uses the smaller Dayton DAEX58FP transducer, which has a fairly low profile and puts out a maximum of 40W. The lower rail uses the much heftier HDN-8, which is weatherproof and *bumps* a full 100W. Daniel wanted to try both transducers out, and so the tuning blocks (the wood parts, which have piano tuning screws in them) needed to be built at different heights.
Incidentally, these transducers are intended to be mounted to walls, floors or furniture to provide a more tactile audio experience. Imagine 100W of bone-shaking bass when you're watching a movie - I'm not sure it would be enjoyable, but I sure would like to try it out.