Where the hell is commonsense in all of this.
It is clear as mud.
The fixed reserve is there to cover unforeseen situations that may require the use of the fixed reserves.
Remember when Ops would actually scrutinise your flightplan and have Flightservice call you with questions when they thought things didn't add up?
I remember having to explain to a Ops man in BN as to why the 1 minute Margin fuel I had on my FP was sufficient. It did not compute that I had 45min Fixed, 15% Variable and 60Min holding. Seeing that I was only going BN-MCY-BN, my reserves were more than my flight fuel.
He signed the FP in due course.
I did remark after he signed the FP that he had no knowledge of what fuel/endurance was actually in the fuel tanks and his signing was just an ACE.