The Group Policy setting for 'Files not cached' (Computer Configuration\Administrative Templates\Network\Offline Files\Files not cached) does not apply to Windows Vista.
Offline files in Windows Vista uses Bitmap Differential Transfer. Bitmap Differential Transfer tracks which blocks of a file in the local cache are modified while you are working offline and then sends only those blocks to the server. In Windows XP, Offline Files copies the entire file from the local cache to the server, even if only a small part of the file was modified while offline.
This behavior is by design.
Because Bitmap Differential Transfer can make the synchronization more efficient, Offline Files in Windows Vista supports all file types. Offline Files in Windows XP could not synchronize large files efficiently; therefore, certain types of files were specifically excluded from the local cache.