Because fax numbers are electronic addresses.
The standard behavior for the Outlook Address book is to display all
electronic addresses (both fax and e-mail). That behavior cannot be changed.
The only way to prevent the display of fax numbers is to store them in a
different field or to disguise them (e.g., precede them with an alpha
character) so that Outlook won't recognize them as phone numbers.
There are also a number of utilities available that can do this for you
automatically. Some are listed here:
http://www.slipstick.com/contacts/nofax.htm