Yes, because the navigation tree shows all the pages while the Select Page dialog shows only the pages that can have children. If you want to move page X under Y then Y must be able to support child pages. And yes, not all the pages can have child pages. There are two types of pages: nested pages and terminal. Terminal pages can’t have children (they are leafs in the hierarchy tree). See http://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/ContentOrganization/#HTerminology .
The blank pages you create from the Create Page wizard are nested by default. There’s a check box to toggle between nested and terminal, but it’s only displayed to advanced users http://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/DocumentLifecycle/#HMove2FRename. The problem in your case is that the confluence importer has created terminal pages. You can “convert” a terminal page to a nested page easily though, by simply “renaming” that page without changing anything except for the terminal flag. Switch your user profile to advanced before that http://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/PageEditing#HAdvancedMode .