737 rudder trimming

[hoody_mcboob]

OK, im assuming this situation will occur in most jet aircraft, as opposed to just the 737, but i dont understand why. You take off get to top of climb and trim the aircraft out with the rudder trim such that the yoke is level. Then you descend disconnect the A/P to fly the approach and landing and quite often the rudder needs to be trimmed back through neutral and in the opposite sense to that used in the cruise. Im not whinging about the effort it takes ;), im just curious as to why this is the case.
Thanks

[BOAC]

Think flaps..............?

[gonso]

Think speed..........?

[CaptainSandL]

Think temperature changes.................?

[ok1]

Think off-center toilet tank.........? :)

[A Very Civil Pilot]

Even a slight difference in N1 can cause a noticable amount of yaw.

[mavrik1]

thrust vortex out of engines, thrust setting again