Given that the fan airflow creates most of the thrust in a high-bypass machine like CF6, the rated thrust is a matter of pushing a specified airflow (about 1400 lbs/sec) through a geometrically-specified fan nozzle annulus.
To pump this much airflow on a standard day, it takes about 3800 rpm. If a hotter day (lower air density), more rpm is needed, and if a colder day, less rpm to achieve rated thrust. Similarly with altitude.
In the engine development testing, enough data are acquired over a range of temperatures & pressure altitudes that the shape (slope, etc.) of the power management curves is well verified. The net result is that the left-hand side of the chart essentially contains lines of
constant fan airflow, and thus constant thrust.
