Same sender listed two different ways.

I have this issue in Outlook with some of my users where the same email senders will appear in two different ways in the receivers inbox.  For instance, in a user's inbox some senders will appear with Last - First name.  The same sender may also be listed in the inbox by the email address.  This means that when you want to search an email from that person, you need to look in two different locations in the list.  First under the first letter of the last name when sorted alphabetically.  The other possition starting with the first letter of the email address.  Why could this be?