The MediaBox defines the area of a page - the actual media size.
However there are other measures as well.
The CropBox defines "the visible region of default user space.
When the page is displayed or printed, its contents are to be
clipped (cropped) to this rectangle and then imposed on the output
When you display your PDF in Acrobat what you see is the area of
the page defined by the CropBox rather than the MediaBox.
Normally CropBoxes and MediaBoxes are identical or practically
identical. However occasionally you may find that they're not.
This may cause your output to appear in a different location
from the one you are expecting.
Indeed your PDF may also impose a TrimBox or a BleedBox. If you're
curious about these there are full details in the Adobe PDF