You have two component's one with a positive height of 0.75" (Speaker face) and one with a negative height of 0.75". The components don't overlap so the 'Merge' setting makes no difference as they don't merge into each other, therefore both of the heights are based off the modelling plane. Hence the overall height is 1.5" made up of the +0.75" component and -0.75" component.
If you want them all to be positive, set the base height for the 'back of speaker' component to 0.75" and then both components will fit within 0.75" material.
Does this make sense?