Apple - username different in Terminal and System Preferences

In Terminal the whoami displays your Short Name. This Short Name is also found as the name of your Home directory.

When you go into System Preferences and look at the user on the left it displays your Full Name.

Mac OS X does not let you easily change your Short Name, but you can change your Full Name from System Preferences.

Refer to this knowledge base from Apple if you wish to change your Short Name. http://support.apple.com/kb/ht1428