No Idea Either,
I had the same chat with one of the senior guys at my well known low cost outfit when I was checked to the left seat.
For all the emails and newsletters discussing unnecessary fuel uplift, nobody has ever called me in to the office for extra uplift cup of tea, not once. My take on this is, give it some thought as to what your extra would really be needed for, don't just take it because it feels good. Otherwise, the PIC's discretion is left alone.
Classic example, if you're going from SYD to CNS on a Gin clear day and wish to take TSV as an ALT, I would use the variable from SYD to CNS and whatever tanker is provided as some of the flight fuel from CNS to TSV, the extra uplift is therefore bugger all and I know that I can touch down at TSV with fixed reserve intact.