Not such a bizarre question really - the info is there if you look for it.
In the C310 the compass weighs-in at 0.6lbs and has an "arm" of 13 inches.
All of this data contained in the W&B record for the aircraft, and updated whenever anything is changed/added/removed.
The instructor is teaching a principle, nothing more nor less.
If you know how/where to get this information and how/where to use it, you will be able to safely do W&B calcs on more "normal" items such as fuel and passengers.
Are you looking in the aircraft's CAA-approved POH or an "abridged" flying-school version?
Have you checked the aircraft's W&B records?