Stewart,
This is because your first button has focus and so it is "highlighted". If any other control has focus the button is not "highlighted".
To prove this you can put two buttons in the form with no other controls. When you run the form the first button will be "highlighted". Click on the other button (removing focus from the first) and you will see that the first button loses focus and the "highlight".
If you want the buttons to not be "highlighted" there must be some other control in the form to receive focus and you must insure that the other control gets focus either by being earlier in the tab order than the buttons or by issuing a Thisform.Whatever.SetFocus() call in the form's Init event.