Overflow box with width as auto

Please resize the viewpoint until the overflow box is 400px in width. Each background square is 25px in width.

Overflow box 'width' auto and resized to 400px

1a.
1b.
1c.

Scrollable box 'width' 825px = ('margin-left' 25px + 'width' (400px x 200%)800px + 'margin-right' 25px~0px).

Overflow box 'width' 400px

2a.
2b.
2c.

Scrollable box 'width' 875px = ('margin-left' 25px + border-left-width 25px + 'width' (400px x 200%)800px + 'border-right-width' 25px + 'margin-right' 25px~0px).

3a.
3b.
3c.

Gecko 1.8: Scrollable box 'width' 775px = 'padding-left' 50px + ('margin-left' 25px + border-left-width 25px + 'width' 600px + 'border-right-width' 25px + 'margin-right' 25px~0px = 675px) + padding-right' 50px.

Gecko 1.9 & Safari 3: Scrollable box 'width' 725px = 'padding-left' 50px + ('margin-left' 25px + border-left-width 25px + 'width' 600px + 'border-right-width' 25px + 'margin-right' 25px~0px = 675px) + padding-right' 50px~0px.

Opera 9.5: Scrollable box 'width' 825px = 'padding-left' 50px + ('margin-left' 25px + border-left-width 25px + 'width' 600px + 'border-right-width' 25px + 'margin-right' 25px = 700px) + padding-right' 50px + mystery 25px.

4a.
4b.
4c.

Opera 9.5: Scrollable box 'width' 850px = 'padding-left' 50px + ('margin-left' 25px + border-left-width 25px + 'width' 600px + 'border-right-width' 25px + 'margin-right' 25px = 700px) + padding-right' 50px + mystery 50px.

the extra 25px in Opera 9.5 is due to the 'border-left' of the overflow box. This does not happen with 'border-right'