I had a guest that cancelled within 24 hours of his arrival due to his mother being taken to the hospital. Airbnb did not charge the guest for the 1st night stay but did pay me based on my flexible cancellation policy.
I verified with the guest that they were not charged otherwise I would have refunded him myself.
Sounds like you experienced the same.
Sometimes when a guest calls Airbnb to cancel Airbnb will reach out to the host to see if you would be willing to accept the cancellation forgoing your cancellation policy. But hosts do not have to agree and then Airbnb will decide whether to refund the guest at their expense. Host is still paid based on cancellation policy.