It is not technically a 'limit' although you simply cannot physically take off at a higher weight than MZFW plus
loaded fuel. The whole matrix is designed to enable a calculation of the 'underload' available to dispatch depending on the limiting factor and only the Take-off and landing columns are relevant to
limits. Box 'C' will not change with either trip fuel, LDG wt or RTOW, only with loaded fuel.
Remember that Wet weight is DOW+Fuel, so as you increase fuel so wet weight goes up, but max allowable payload will initially remain the same. Only when the 'ZFW limited weight' overtakes the other two weights as you pile on the fuel (or trip fuel reduces) do we actually hit a
real limit, be it RTOW or LDG.
Its just the way it has been done for years! Clear as mud? If it is still confusing, just accept that Col C is not really 'limiting'.
. Take a few load-sheet blanks and run some different scenarios.