Scanning 8.5 x 11 changes into 8.25 x 10.75. Why?

When I scan an 8.5 x 11 (letter) sheet the final scan page size says 8.25 x 10.75. Why?

This is my set up.

Adobe version: Acrobat Standard 9

Printer: Brother MFC-7460DN (USB directly connected to computer)

Computer: Dell XPS 8900 running Windows 7 Professional (64bit)

I have gone into the Adobe settings. Create --> PDF from scanner --> Configure presets --> Paper size "Letter" "8.5 in x 11 in"

It does not make any difference if I scan via the document feeder or the flat glass surface. Both end up as an 8.25 in x 10.75 in.

How can I have my final scanned document be the correct size (8.5 in x 11 in)?