1) The fixed margins displayed in the LaserWriter 8.0 Page Setup dialog are "standards" created by the Apple Imaging hardware group and these same standards apply to all LaserWriters since the LaserWriter IINT, with a few exceptions. All of the margins are based on what the maximum imageable areas are for the various print engines. When a different PPD (PostScript Printer Description) is selected for a different printer, these values may change to reflect the imageable area for that print engine.
2) In terms of your question with "Larger Print Area", the margins displayed are in fact the minimum margins that create a boundary for the maximum imageable (or printable) area. NOTE: Even though Larger Print Area is selected, the maximum printable area may not be achieved unless an application specifically supports those minimum margins.