Autocad lets you specify two areas for each size of paper - the paper size, and the plot area. Actually, the plot area is defined by specifying the paper size and the size of the border on the top, bottom, left, and right of the sheet. The border is a no-plot zone - nothing will be plotted there, and anything you plot will be cropped as it is sent to the plotter to fit inside this area. The actual plot area is listed on the plot dialogue box as "printable area" underneath the paper size.
The default settings for most plotters include every paper size twice, one of them called "Oversize". In the page definition, the "Oversize" setting has all borders set to 0, while the other definition has the borders set to some non-zero value. If you use "plot to layout", you'll see the plot area indicated by a dashed line on the layout.
You can adjust all these definitions in the custom settings of the plotter (pc3 file).