Cannot change owner and group of file mounted disk image
If the disk is mounted to ignore file ownership, the owner is always reported as the current user, and cannot be changed. Attempting to change the owner will not produce an error message, but simply fail silently.
In the Finder, select the mounted disk corresponding to the sparse image in question and Get Info (⌘I). At the bottom of the pane that opens, see if it looks like this: (i.e. the Ignore ownership on this volume box is checked)
If you uncheck that box, you should get the behavior you seek.