Engine runs cooler with heater core bypassed. Why?

1991 L03 car. Nothing to add. Heater core developed a small leak, so shunted the hose coming off the back of the motor to the stub off of the radiator.
Used to sit at or above the 180 degree mark on the gauge, now sits a line below it. Car runs better overall. Less apt to ping in the hot weather.

What happened here?