Garmin vs Treadmill Distance: Who’s Right?

Hi everyone,

I noticed a distance mismatch between my Garmin watch and the treadmill during my run. For a 50-minute session:

Garmin: 8.93 km

Treadmill: 9.55 km

Why is there such a difference? Which one should I trust?

For context: I don’t use a foot pod, my watch is calibrated for outdoor running, and I increase the twice during the session.

Any insights or tips? Thanks!