I suspect I was puzzled by Scale because I was expecting it to be more complicated than it actually is and do all sorts of clever adjustments to make things fit on different size screens.
In fact looks as though at layout time, having selected the layout file that best matches the device the Top, Left, Width and Height for each control are multiplied by a factor that seems to be StoredLayoutScale/ActualDeviceScale. Is this correct?
EDIT :- Or more likely ActualDeviceScale/StoredLayoutScale which better matches what I see happening.