Does anyone have any idea why the 'get info' pane of the Macintosh HD is displaying alternate information to the 'storage' pane under the 'about this mac' window?
Not exactly. When you have Time Machine turned on and you delete a file and empty the trash, that file does not really get deleted. It gets moved into a hidden /.Mobilebackups folder and it stays there even after the next backup has run. Deleted files will just accumulate there until your drive starts to become full or you need the storage space, then the OS will start to clear off some of those files.
These are called "local snapshots" and you can read more about it at this link.
The space used for this is not counted in Finder is the reason you are seeing that discrepancy. Totally normal.