Why does layer order affect objects x-y axis (in a stacked group)

I've just noticed that within a (stacked group) if I change the layer order of an object, the x-y axis changes also.

For example if I have three rectangles (green, blue & pink rectangles) in a stacked group with the pink rectangle being the last layer (backward-most layer). 

If I moved the pink rectangle upwards in the layer panel, it will move upwards in the stacked group on the artboard.

This doesn't happen in regular groups