if the volume is created as extended then WXP can then be installed. Maybe the systems you have seen were done this way.
Quote from <A HREF="http://www.mcmcse.com/windows_xp/guides/diskmanagementtool.shtml" target="_new"> here </A>
" When basic volumes are converted to dynamic they may or may not have an entry in the partition table depending on whether that volume was a system or boot partition. If the volume that was converted was originally a system or boot partition it retains its old entry in the partition table. You can install Windows XP Professional on that volume, however, you will be unable to extend it. If the converted volume was not originally a system or boot volume it will not have the old partition table entry. You won't be able to install Windows XP Professional on the volume, but it will be possible to extend it."
A great many people think they are thinking when they are really rearranging their prejudices.