it's probably different reasons for each situation. on our trucks they need room for exhaust aft of the axle on the right and the fuel tank forward of the axle on the left. i also vaguely remember reading about that setup helping with dampening spring wrap. most shocks dampen differently going up and down in travel; if you mount them on opposite sides, they offer equal dampening in regard to spring wrap (one is going up while the other down).
yes, i'm guessing