Well as nobody else is going to challenge the rationale ...
This is the start of a slippery slope. A LATCC / NERC ATCO is paid according to their grade not what they do each day. Using your rationale do we see different rates for a "normal" day in the office versus the Ops room ? Lets take that a stage further. Do those with Clacton validations get more than those with LUS validations ? Or if you are multi valid what about paying you only for what you do on each shift ? work a busy sector and push through the traffic = bonus pay for the day !!