When you add a Plane to a Line, and you edit the position of the Line with the Part-Editor or with a Dimension in a Design Set the Plane does not move to the new positon of the Line.

To get the Plane to move, you must edit the Plane in the Plane dialog, change an angle say, and then close the Plane dialog

So it is an update thing.

This is fairly important because KOSME Co wants to parameterize their models as much as possible, and this offset is key to one of their models. They want to parameterize it so that their technicians can use the model at any time with a design set.