Why do the upper ~18 notes of my Kawai digital/hybrid piano sustain without using the damper pedal?

The dampers of an acoustic piano span almost the full width of the keyboard. However, the topmost ~18 keys (approximately 1.5 octaves) do not incorporate dampers, as the shorter strings for these treble notes decay quickly and therefore do not need to be dampened.

As a result, the strings of these ‘undamped’ treble notes are free to vibrate in sympathetic resonance with those of lower keys – independently of the damper pedal position – helping to enrich the sound with additional harmonics and tonal colour.

Kawai digital and hybrid piano reproduce this same behaviour, therefore the topmost ~18 notes of the keyboard will continue to sustain for longer than the neighbouring notes when the damper pedal is not pressed.