If your report includes a subreport, the width of the subreport could push the boundaries of the body if subreport and hierarchy are allowed to grow.
I had a similar problem arise with a subreport that could be placed in a cell (spanning 2 columns). It looked like the span could contain it in the designer and it rendered fine in a winform or a browser and, originally, it could generate printer output (or pdf file) without spilling over onto excess pages.
Then, after changing some other column widths (and without exceeding the body width plus margins), the winform and browser renderings looked still looked fine but when the output (printer or pdf) was generated, it grew past the margins and wrote the right side of each page as a 2nd (4th, etc.) page. I could eliminate my problem by increasing colspan where the subreport was placed.
Whether or not you're using subreports, if you have page spillover and your body design fits within the margins of the page, look for something allowed to grow that pushes the width of the body out.