On the B742 and B744 and especially on the B748 I believe it has to do with the differing thermal effects on the steel control rod holding the rudder surface in position vs. the aluminium rib structure. Generally as the aircraft structure cools in climb and cruise it pulls the rudder slightly left requiring correction with right rudder trim, and the opposite occurs in descent.
If the rudder control architecture is similar on all Boeings then this could be it?