Magnus,
I think the difference is that Stein provides an avionics harness that attaches to the forward end of the common fuselage harness. The avionics harness, which they custom-make for your specific application, has the additional length for routing behind the panel to all the various equipment.
My common fuselage harness was long enough to come up behind the sub panel and not much more. The avionics harness had plenty of extra length. If someone is using an avionics source other then Stein that would be an issue to address with them.
I shipped my fuselage harness to Stein so they could add connectors to the forward end and verify everything worked with the avionics harness. If I could do it over again I would have just gotten the fuselage harness direct from Stein and saved on the extra shipping costs.