My right and bottom margins of my maps are always outside of the printable area even when I make the guidelines to the maximum printable dimensions and snap the map to them. This doesn't happen to any of my coworkers and it only happens on my computer with the same settings as them.
I'm not exactly sure of the problem so I'll suggest a couple of things. First, if you are using an HP wide format printer (and maybe there is something similar on others) you might want to set the margins to "Small" in the Printer Properties - Paper/Quality - Paper Options - Margins or something similar depending on your driver. This maximizes the print margins. Second, I always set my layout size and location manually in the Data Frame Properties - Size and Position tab keying off the left bottom anchor point. So maybe your settings for either or both of these differ from your co-workers? I you have a different default printer that can impact your default settings too.