the position of a model (instance of an stl file) is basically the offset of the models's origin to the center of the build table.
When you add an stl, the offset will be 0 and you define the origin of the model to be the center of the bounding box of the model.
The center is reestablished every time, the model(stl) is loaded.
This leads to the problem, that when you make an change that effects the bounding box of the model(stl), all it's supports will have to be redone regardless of whether the change would have required this.
If you stored the value of the initial bounding box (when the stl was added) and used that instead of recalculating the origin every time, that problem wouldn't occur and that would save a lot of frustration for people who use lots of supports.