Hi Dan

It's a setting in the selector tool infobar called Scale line widths. If you set it to on, the width of selection will include any outlines and as you have found vice versa.

I'm still using an old version of Xara so it might be in a different position now, but in my version it is a 'small arrow with a backslash' icon next to the flip vertically/horizontally icons ... towards the right-hand side.

Regards

Su