It is a lot of power to a Supervisor.
Supervisor can bias the 2 minute drills to favor his long term 7 friends and throw that new guy that transferred in 4 months ago under the bus. Can also drop anyone that the Supervisor sees as a threat.
The only situation where this is not possible is when a HiPo gets placed into a group. The Supervisor get a phone call or a face to face discussion not on the calendar and is instructed to place the HiPo at the top of his least, embellish the HiPos 2 minute drills, and to not mention any catastrophies the HiPo caused.
I once saw a Supervisor that had 3 direct reports put a very young id--t HiPo below the other 2 normal non HiPo directs and during the two minute drill explained his complete disappointment in the HiPos performance. That Supervisor got in a lot of trouble and dropped in the ranking for not telling lies in the ranking meeting. Integrity and honesty doomed that Supervisor’s career. I knew the HiPo and she was truly useless, but that truth is forbidden to mention in a ranking meeting for a HiPo.