That's the legal answer for Ontario, and addresses the VIN branding requirements once a vehicle is totaled, but it doesn't address the base question of why insurers and repairers total vehicles at different rates.

If the regulation removes any objectivity then why the different rates. Perhaps it is in the motivation, repair vs replace, timing constraints, etc, etc.

We all keep seeing easily repaired vehicles towed away to a salvage auction - and it is helpful to occasionally ask "Why?"