I don't know where the differing parts are located exactly, but somewhere in there the wagon speedos are calibrated to a .960 ratio, and the sedans to a .980 ratio, due to the different tire sizes. The gears that key the speedo to the rear end ratio are located on the tranny in the pre-'86 models with mechanical speedos, and on the '86- models, the electronic pickup is right on the diff.

I guess Volvo/VDO didn't want to go to the expense of creating a near-duplicate set of tranny-mounted speedo gears for the wagons, so they figured doing the final adjustment internally was best.