the man page for hdid explains the problem:
Beware that an image you have created and attached is considered an unknown removable device. For HFS filesystems on such a device, being unknown to the system means that the on-disk ownership of files and directories are ignored by default. On 10.2, they were dynamically replaced with the owner of /dev/console and the group unknown (gid 99). On 10.3, the group remains unknown, but the owner is whoever is currently accessing the file (joe sees that he owns the file when he looks; mary sees she owns it whenever she looks). Owners can be enabled for a particular volume permanently (see disktool/”get info” in the Finder) or temporarily (see EXAMPLES section of hdiutil(1)). Aside from whether owners are enabled, being removable means that disk arbitration will mount any volumes with special options such as nosuid.
However, this does not identify the original file ownership settings nor explain the criteria for those files that did not get remapped.