iaf_22,
A lot of folk forget that we are comparing oranges with apples .. Certification Design Standards prescribe a bunch of things including lines in the sand minimum performance criteria for various conditions.
In the real world there are many occasions when the aircraft's weight has to be restricted to meet the requirement of the day ... having to schedule RTOW data for departure ... having to restrict TOW to make an OEI cruise capability to match a high enroute terrain requirement... or having to address a host of other limiting requirements along the way ... having to take a lesser payload to permit enough fuel to be uplifted for the sector distance ... etc., etc ...