When I first approached this question, I was under the impression that it was about how the document looked on the screen, mainly. In any event, I have been reading about the infamous printer driver, which seems to be popular around here for an answer, but I cant convince myself of such. In my experience of 20+ years working with these things, I can tell you that I have right now (2011) certain Word documents that were conceived 10+ years ago (standard margins, of course, nothing fancy) in earlier versions of Office, different computers, different Windows OS, different languages, different countries, different printers (emphasis added), and today, these documents look just the same as 10 years ago, in new computers, new systems, latest Office suite and several new printers. The font used was standard Times New Roman, so everything is exactly where its supposed to be, no matter what. (Again, all the time using the regular margin as a new document offers it.)

Now, adding to my initial answer, historically Microsoft Word has included a Compatibility tab, which can be found in Tools/Options, and has a plethora of options to adjust things in the document. If these options are enforced, things might look different in the document on-screen and/or in-print. For example, I remember back in the day of the dot matrix printers when we had to check some of those options in order to have the tables printed out right (the same tables that looked perfectly fine in the screen).