Who cares? It doesn't affect the operation of the computer in any way if you don't use the server app.
Actually, Software Update will still think it's Server, but there's an easy way to fix it...
In System/Library/CoreServices/ there's two files... ServerVersion.plist and SystemVersion.plist... This appears to be where it gets the info for the "About This Mac" dialog, but it also effects what Software Update does... If ServerVersion.plist is present, it will look for updates for Server... If it's not there, then it moves on to SystemVersion.plist and SW Update will then operate on Client OS updates. There's also, IIRC, some changes in System Prefs, too, between the two.
For the most part I'm sure that there's really no difference on the OS update for either one, though.