The only thing that's difficult to understand is how the faker went to such trouble to get various details correct, and then made the bonehead mistake of not using a font like Courier that would emulate a 1970s-vintage typewriter.
The forger didn't go to much trouble because it wasn't necessary, from the forger's point of view. The person who wrote it was intimately familiar with the controversy surrounding Bush's ANG service, had military experience him/herself, and came from background where the blogosphere did not exist (so it would never occur to the forger that an image of the document would be widely available to skeptical eyes.)
Intimacy breeds casualness. The forger relied on his/her already well-established knowledge to write the text. The forger thought the immediate response to the memo would focus on the content and not on the appearance.