Two often-overlooked factors have an outsized effect on how convincing a digital piano actually sounds in person, beyond sample quality alone.
Built-in speakers
Speaker size, power and cabinet design vary enormously between models. A budget instrument's small speakers can make even good piano samples sound thin or boxy, while a well-built cabinet with larger speakers lets the same underlying sound quality project with real depth - this is one of the few genuine advantages of buying an instrument in person or asking for an unedited video rather than judging specs alone.
Simulated string resonance
On a real acoustic piano, pressing the sustain pedal lets undamped strings vibrate sympathetically with whatever is being played, adding subtle overtone complexity. Higher-tier digital pianos simulate this effect (sometimes called "damper resonance" or "string resonance") digitally; budget models often skip it, which can make sustained pedal passages sound comparatively flat and simple.
Neither feature is something you can judge from a written spec sheet the way you can compare polyphony numbers - both are worth actually listening for when trying an instrument, ideally through headphones as well as its own speakers.
