From fileinfo.com:
RTF: Rich Text Format File
Common text file format that supports "rich text;" may include several types of text formatting, such as bold type, italics, different fonts and font sizes, custom tab settings, etc; may also support images saved within the text file.
RTFD: Rich Text Format Directory File
Rich text file similar to a .RTF document, but may contain attachments, such as images, PDFs, fonts, or other files; originally developed for the NeXT operating system and later moved to Mac OS X.
A test of some sample documents showed Pages will use the .rtf extension if the (word processing) document contains only formatted text, but as soon as a graphics object is added, the extension used is .rtfd. Pages Layout type documents always contain objects (all text is contained in text boxes), so I suspect they will always export as rtfd.
Regards,
Barry