Let me give you the first rule of corroboration, be it in the court room during cross examination or during an investigation:
As humans were do multiple things every day. If I ask anyone what they did a month ago and if the answer I get is flawless, with details, then there is a reason to be suspicious and this concludes that either the person is lying or has been tutored.
This is the same way how the IRCC phone call works. If your supervisor is asked about an employee, and he simply confirms that the employee works at the office, and performs x, y, z duties, that is sufficient. IRCC does not expect them to narrate the entire job duties. There may also be an instance where the supervisor does not even remember the employee by name, and may have to check his database to confirm, this is perfectly normal.
Now, if the supervisor narrates the exact things written in the reference letter, then there is a big red flag.
I got you got the crux of this.