When creating a new user, the old-core XWiki.createUser(...) method still sets the user's profile document content to:
...which, for backwards compatibility reasons, has the effect of overriding the XWikiUsers class configured sheet and rendering the specified sheet instead. In this case it is the same sheet, but rendered in a different context.
The problem it creates is that users without script rights might not be able to view their profile since the XWikiUsersSheet document would be rendered with the rights of the user document instead of being rendered with the rights of the sheet document.
This method is used by registration and possibly in other places as well and should have been deprecated when we`ve introduced the sheet displayers.